Matrices: partial implementation of the Moore-Penrose pseudoinverse.

Added two methods: one to calculate the pseudoinverse, and another to apply it
in the solution of Ax = B.  The pseudoinverse calculation works only for full-
rank matrices and zero matrices; rank-deficient matrices will be more
complicated, possibly requiring a complete symbolic implementation of singular
value decomposition.  Until then, this should work well enough (at least for
me!).

See https://en.wikipedia.org/wiki/Moore-Penrose_pseudoinverse