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
Matrix pinv: switched arbitrary matrix to dummy symbols, added examples.

At Aaron Meurer's suggestion, changed the default arbitrary_matrix of
pinv_solve to contain dummy symbols instead of regular symbols, and added
examples to both pinv and pinv_solve.
Merge pull request #2571 from Tempel/matrix_pseudoinverse

Matrices: partial implementation of the Moore-Penrose pseudoinverse.