Pab/library fixes

.. multivaiate polynomials added.

This should be most of the remaining algebra files that were not built.
The main motivation for doing these was that #87  requires much stronger 
conditional checking, and the existing library is a little lax on that front. So,
completing the library before a bunch of random conditionalisation changes seems a 
good idea.

One minor issue - there's a combine function missing in action - referenced 
by the multivariate gcd implementation, but not defined anywhere.  I've stubbed 
it in the hope that an implementation will show up.