Adds distribution data for N-Body distribution

[HunterBelanger]

This PR addresses the issue that Wim mentioned in #16 , and adds the information about the representation of the distribution data given for the fraction of the max energy quantity (referred to as T in the document).

Originally, it was stated that the provided distribution was used for sampling T, but the equation provided was not completely consistent with this. I modified the distribution to be consistent with what the analytic form should be for PDF(T). If you want, I can change this be be the distribution for outgoing energy (as presented in the ENDF manual Equation 6.21).

The information about the distribution data can be found in acefc.f90 near line 7300, and acefc.f90 line 13265 in NJOY2016.