Add Support for Sparse by Sparse Matrix Multiplication

[RoyiAvital]

Hi,

Any chance to add matmat() for sparse matrices?
It is implemented by oneapi::mkl::sparse::matmat.

Is is for the multiplication of 2 sparse matrices.

[alyst]

It is in the works. See #50

[RoyiAvital]

@alyst , That's great!
I don't see matmat() in your code. Is spmm() a wrapper of it?

[alyst]

I think it is rather the opposite, spmm() and spmmd() are low-level C/Fortran Sparse BLAS calls, and matmat is a C++ wrapper.

[alyst]

Closed by #50.