Void_distribution class Update 

- 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.
- Added Pickle constants file, for WMAP7 powspec class variables ('void_init.p')