Sparse backwards and forwards substitution

[mikera]

We should provide backward and forward substitution algorithms for sparse matrices.

See: #29 for motivation

[mikera]

@moygit might be an interesting one for you given your work with the sparse matrices?

[moygit]

Sounds good, I'll add it to my list. Might be a little while, though: I'm looking into speeding up some multiplications right now, and also busy with a few other things (non-Vectorz).

[mikera]

Sure - no hurry!