Add optional matrix::rank() algorighm selection.

Add optional matrix::rank() algorighm selection.

Before, matrix::rank() was hardcoded to use Bareiss elimination,
which for some matrices is way too slow (and I had to work around
this limitation in my own code). I propose adding an argument that
will allow users to select which elimination scheme to use. The
argument is the same as matrix::solve() takes (so, solve_algo::*).

I've tried to mimic how a similar argument was added to
matrix::inverse() by keeping the current set of rank() functions
and defining separate ones with the 'algo' argument, with the
old ones calling the new ones with solve_algo::automatic. This
is instead of using default argument values.

I've also put the code that makes the automatic selection of
the elimination method into a separate function so that both
matrix::solve() and matrix::rank() could share it. There's a bit
of similar code in matrix::determinant(), but since the self
of determinant algorithms is different from the elimination
algorithms, it's not clear if these two could be unified.