New memory management policies, new SketchingOperator concept, redesign of DenseDist and SparseDist, lots of docs

[rileyjmurray]

This branch was originally created to expand source and web documentation. However, as I tried to write those docs I found myself questioning many previous design decisions. (It turns out this is a trend.)

Anyway, the PR is now gigantic in scope. I've tried to break down changes into categories.

Changes to source documentation

• Introduced macros for opaque matrices and vectors in rst-compatable source code comments (\mtxA,\mtxB,\mtxX,\mtxS,\mtxx,\mtxy). Right now those macros render their arguments in bold, but we could easily switch to a different style.
• Removed most bracket-based indexing notation like \mat(A)[i, j] in favor of mathematically typeset subscripts like \mat(A)_{ij}.
• Removed (the vast majority of) rst macro directive definitions from source code comments.
• Complete rewrites of DenseDist, DenseSkOp, SparseDist, and SparseSkOp docs.
• Complete rewrites of COOMatrix, CSRMatrix, and CSCMatrix source docs to match the style of DenseSkOp and SparseSkOp.
• Added source docs for the Axis enum (renamed from MajorAxis).
• Added source docs for the SketchingOperator concept.

New features and identifiers

• Change the definition of LASOs so they actually implement LESS-Uniform (with Rademachers as the scalar sub-gaussian distribution).
• Made the SketchingDistribution and SketchingOperator concepts apply meaningful constraints.
• Extend sample_indices_iid_uniform to support generating Rademachers in addition to the indices themselves, much like how repeated_fisher_yates works. Exposed an overload that matches the original signature.
• Add a fill_sparse overload that's analogous to the many-argument fill_dense overload.
• De-overloaded the verbose fill_dense and fill_sparse into new functions: fill_dense_unpacked and fill_sparse_unpacked_nosub. The latter has the nosub suffix because it doesn't have all the parameters needed to select a submatrix, and I'll eventually want a fill_sparse_unpacked that qualitatively matches fill_dense_unpacked.

API changes : types

Adopted a consistent policy on semantics of memory ownership. This included making sure all sketching operator and sparse matrix classes have non-const own_memory members.

Added isometry_scale member to SketchingDistribution, DenseDist, and SparseDist.

Added dim_major and dim_minor to DenseDist and SparseDist. Deliberately keeping this out of the SketchingDistribution concept.

RNGState

• Defined a DefaultRNG type, equal to r123::Philox4x32. RNGState's default RNG template parameter is now RNG = DefaultRNG.
• Add static_assert that counter length for RNGs is >= 2. Needed for Box-Muller transforms and many other places.

SparseDist

• Changed initialization order.
• Add full_nnz member
• Added a constructor for SparseDist, and replaced many instances of defining SparseDists with initializer lists. The construct has vec_nnz=4 and major_axis=Short by default.

SparseSkOp

• Add nnz member
• Remove known_filled member from SparseSkOp; the old check "!S.known_filled" is equivalent to "S.nnz <= 0".

SparseMatrix classes

• Remove reserve instance-methods.
• Add reserve_xyz free-functions to replace the reserve instance-methods.
• Removed some (mostly) redundant constructors.

Important changes that don't affect the public API:

• Renamed MajorAxis enum to just "Axis".
• Removed Axis::Undefined and ScalarDist::BlackBox.
• Make a (private!) helper BLASFriendlyOperator datatype, so that I don't have to coerce DenseSkOp into supporting submatrices directly.

API changes : free functions

• LSKGES and RSKGES no longer call fill_sparse on a user-provided unfilled SparseSkOp. Instead, we make a temporary shallow-copy, fill that copy, and delete it before returning. This is in preparation for when LSKGES/RSKGES only generate submatrices that are needed for the operation at hand, and it follows the same memory-allocation/ownership semantics of dense sketching ([L/R]SK[GE/SP]3).
• Template sample_indicies_iid and sample_indices_iid_uniform to allow general signed integers, rather than only int64_t.
• Removed the ability of spmm and sketch_sparse to operate on submatrices of sparse data matrices. This basic functionality is still preserved by left_spmm and right_spmm in the former case and lsksp3 and rsksp3 in the latter case.
• Removed the "val" argument from the overwrite_triangle utility function. I was only using this function with val=0.0.
• remove has_fixed_nnz_per_col(S), since it wasn't used.
• Moved several utility functions into the main RandBLAS namespace.
• Significantly modernized print_colmaj.

Templating conventions

• Changed templating in sketch_sparse to avoid nested templating. By nested templating I mean something liketemplate <typename T, typename RNG> and having an argument of type DenseSkOp<T,RNG>, while more "flat" templating would use template <typename DenseSkOp, typename T = DenseSkOp::scalar_t> and then have arguments of type T and DenseSkOp.
• Changed templating functions that only accepted sparse sketching operators but templated as SKOP. They now template as SparseSkOp (which in a technical sense makes no difference, but does make clear that it's supposed to be a SparseSkOp<T,RNG> for some parameters T and RNG.
• Started templating RNGState in function calls as typename state_t = RNGState<DefaultRNG>.

Web docs

• Actually wrote the tutorial on selecting parameters for DenseDist and SparseDist!
• Added a "FAQ and Limitations" page to the web docs. Includes discussion of C++ idioms we do/don't use.

Other

Added a utility script for converting my awkward reStructuredText source code docs of function arguments into doxygen format. I thought the doxygen format might be viable since I tweaked the CSS, but it wasn't. I'm keeping the script in case I want to revisit this later.

Increased the size of allowed error messages in randblas_error_if_msg.

Added tests for LSKSP3 and RSKSP3.