[ENHANCEMENT] Define BlockSparseArrays.blocktype(randn(2, 2))

[mtfishman]

Define BlockSparseArrays.blocktype for dense arrays. Currently it outputs:

julia> using BlockSparseArrays: BlockSparseArrays

julia> BlockSparseArrays.blocktype(randn(2, 2))
ERROR: MethodError: no method matching blocktype(::Matrix{Float64})
The function `blocktype` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  blocktype(::Type{<:Union{var"#s41", var"#s40"} where {T, N, var"#s41"<:BlockSparseArrays.AbstractBlockSparseArray{T, N}, var"#s40"<:Union{Base.LogicalIndex{T, <:BlockSparseArrays.AbstractBlockSparseArray}, Base.ReinterpretArray{T, N, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s14"}, var"#s14"}} where var"#s14"<:BlockSparseArrays.AbstractBlockSparseArray, Base.ReshapedArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}}, SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}} where var"#s15"<:BlockSparseArrays.AbstractBlockSparseArray, SubArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, Base.ReshapedArray{<:Any, <:Any, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, var"#s16"}} where var"#s16"<:BlockSparseArrays.AbstractBlockSparseArray, LinearAlgebra.Adjoint{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Diagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.LowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Symmetric{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Transpose{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Tridiagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitLowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitUpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, PermutedDimsArray{T, N, <:Any, <:Any, <:BlockSparseArrays.AbstractBlockSparseArray}}}})
   @ BlockSparseArrays ~/.julia/dev/BlockSparseArrays/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl:88
  blocktype(::Union{BlockSparseArrays.AbstractBlockSparseArray{T, N}, Base.LogicalIndex{T, <:BlockSparseArrays.AbstractBlockSparseArray}, Base.ReinterpretArray{T, N, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s14"}, var"#s14"}} where var"#s14"<:BlockSparseArrays.AbstractBlockSparseArray, Base.ReshapedArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}}, SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}} where var"#s15"<:BlockSparseArrays.AbstractBlockSparseArray, SubArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, Base.ReshapedArray{<:Any, <:Any, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, var"#s16"}} where var"#s16"<:BlockSparseArrays.AbstractBlockSparseArray, LinearAlgebra.Adjoint{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Diagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.LowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Symmetric{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Transpose{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Tridiagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitLowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitUpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, PermutedDimsArray{T, N, <:Any, <:Any, <:BlockSparseArrays.AbstractBlockSparseArray}} where {T, N})
   @ BlockSparseArrays ~/.julia/dev/BlockSparseArrays/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl:87

Stacktrace:
 [1] top-level scope
   @ REPL[34]:1

That will be useful in generic code. Like BlockArrays.jl, we should in general use a convention that non-blocked arrays act like block sparse arrays with a single stored block.