allow to parameterize luminosity functions in "natural parameters" instead of mean_luminosity

The mean_luminosity is not a good property to characterize powerlaws or broken powerlaws. A more natural choice would be Lmin, Lmax, gamma (or Lmin, Lmax, Lbreak, gamma1, gamma2).

The current design puts a lot of emphasis on the mean_luminosity

FIRESONG/firesong/Luminosity.py

Line 232 in 9955d4c

def get_LuminosityFunction(mean_luminosity, LF, **kwargs):

and performs the transformation to the natural parameters implicitly e.g. here (powerlaw)

FIRESONG/firesong/Luminosity.py

Line 182 in 9955d4c

self.Fmin = (self.index+2.)/(self.index+1.)*self.mean

Suggest to rework the interface to the underlying PDFs such that the user can specify the natural parameters directly.