Feature/initial bhs

[nmdickson]

Adds a new class to evolve_mf which is meant to focus the analysis of the evolved mass functions only on the initially created black hole populations, essentially by stopping the evolution once all BHs are created and simplifying the differential equations involved correspondingly to speed it up.

Overall just provides a much simpler and quicker way to look at the BH populations created by the mass functions.

[nmdickson]

This also provides a fix to a pretty important bug: The IMF normalization constants were not being correctly computed, as there was a missing mass term in the 3-component equation. This meant that, in the past, the initial numbers of stars never actually added up to N0 as they should have, and depending on the values of the given IMF slopes could have had nuanced small effects on the proportion size of each of the mass regimes.

This is fixed now, in 88e322d