Furlanetto & Piran Galaxy / Halo underdensity formalism added

- Fitting function to galaxy underdensity / linear void underdensity relationship is still not complete; requires input of a given void radius for each fit. 
- Relationship between survey depth magnitude and minimum observable mass not understood; the paper quotes Blanton et al. (2003) however I can't see a clear definition of the realtionship. Make comparison with figure [2] difficult. 
- When the minimum observable mass is set to 10^10 M_0 the code breaks down; prevents comparison with m_h > 10^10 M_0  fit in figure [2]