suzerain: #2164 change perfect_decl.py generation

Older version did not include derivative expressions,
which turn out to be necessary.  Breaks generation into a
two step process.  The first is SymPy magic, and the second
is turning the SymPy magic into a form useful for computation.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45395 0bd85f0b-c151-43b9-a1e9-27231104551f