Added Furlanetto & Piran formalism for galaxy underdensity. Created a simple void class to aid writing of void functions (keeps void_barrier and collapse_barrier variables consistent). Pickle functionality added to speed up development; saves powspec class variables to avoid recomputation.
Pickle constants file, for WMAP7 powspec class variables
Merge pull request #12 from polyphant1/void_evs

Void_distribution class Update