Monomial algebra improved, preliminary Posynomials

Monomials:
  - using the set datatype for varlists sorts them and removes duplicates
  - added a exps attribute to store the a values
  - added multiplication and powers, and tests for both
  - equality now based upon equal str represenations so as to be
      preserved across sorting

Posynomials:
  implemented a preliminary Posynomials class, which
    - can handle monomial algebra
    - can output string representations