No support for sparse matrices

[kw]

Cousin to Issue #5. I personally do not use sparse matrices on a regular basis, and so this is low priority to me. Since sparse-ness is a property any numeric can have rather than a specific class that sparse matrices are members of, a sparse leaking into pymex could potentially cause things to fail in a spectacular fashion. I should probably add it as a virtual class somewhere in mro.m so that it can associate with a specific wrapper class.

SciPy has sparse matrix support, so it's entirely possible that their formats are compatible. Or perhaps like complex arrays they're implemented entirely different.

I half expect Tim Davis or one of his students to patch this before I do.