Introduce AbstractSparseFactorization and AbstractDenseFactorization

[j-fu]

Both are subtypes of AbstractFactorization.

This allows to set default_alias_A=true for sparse factorizations (which just cannot overwrite A anyway). This allows to create the cache parametrized with the original matrix type and we can use reinit!(cache;A) or cache.A=A if A is an AbstractSparseMatrixCSC, like with Krylov solvers.

Before, due to #525, these methods would require a SparseMatrixCSC, even if the Problem was created with a AbstractSparseMatrixCSC.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.