[Bug-gsl] Please implement typical functionality for sparse matrices

[David Cortes]

The function for matrix multiplication for sparse matrices
(gsl_spblas_dgemm) currently only works when both inputs are in column-
compressed format. It would be straightforward to expand it to cover
also row-compressed matrices (without transposing them).

As well, the spmatrix module supports convertion of
coordinates/triplets formats to row- and column-compressed, but it
doesn't support convertion from a compressed format to
coordinates/triplets.

This makes it also problematic to perform basic algebra on matrices
which might later get multiplied.