PowerLawComponentMass normalizations

finished debugging PowerLawComponentMass normalizations and added more unit tests to cover those cases. Below, I've included exmaple results from the unit tests that show the distribution behaves as expected. This includes the addition of a PDF note deriving the normalization (in case we ever need to revisit this).

mass1_pow == -1 ; mass2_pow == -1

mass1_pow == -1 ; mass2_pow != -1

mass1_pow != -1 ; mass2_pow == -1

mass1_pow != -1 ; mass2_pow != -1