Review issues

I'm going to update the top of this issue with check boxes, for more involved cases, I might make subissues.

• undeclared dependency - gwpopulation_pipe
• PowerlawPlusPeak_PrimaryMass docstring refers to slope, the parameter is alpha = -slope.
• PowerlawPlusPeak_PrimaryMass assumes log mass if an array of mass is passed, but that contradicts the docstring and data is not exponentiated.
• BrokenPowerlawPlusTwoPeaks_PrimaryMass assumes the opposite for log mass.
• trunc_gaussian: I had to be more careful about the normalization on this in gwpop, the version implemented here should not be trusted if the mean isn't contained within [lower, upper].