Gem tabulation

[pbrubeck]

Tabulate unknown points directly to gem, as opposed to first tabulating to sympy.

The gem-only approach is significantly faster:

pytest test/FIAT/unit/test_macro.py -k sympy
=============================== 6 passed, 233 deselected in 14.77s ===============================

pytest test/FIAT/unit/test_macro.py -k gem
=============================== 6 passed, 233 deselected in 1.31s ================================

[rckirby]

I like the speedup and reduced dependency on sympy. I have a few questions