Symbolic gaussian elimination prototype (#128)

Components of this prototype have been incorporated in the kinetic schemes PR #145, but that PR does not retain the more extensive numeric validation code nor does it perform the operation or fill in accounting that is reported in the demo for the evaluation of various pivot choice schemes.

I propose that we keep this demo application for its utility and code base when we revisit this problem, particularly for the investigation of:

the numerical behaviour for larger matrices (limited dynamic range will likely force periodic normalization of temporaries);
better pivot selection schemes for reduced fill-in;
symbolic computation that will arise in higher order implicit schemes.