[BUG]: f_chabrier

- Fixed a bug in f_chabrier which caused the likelihood to not converge
  to the correct value.
- Code had     #knorm = 1. / dN_arr.sum() , so was normalized to the
  entire range of masses of the isochrone (mass_min_global < M < mass_max_global),
  whereas it should be normalized to the observed range of masses,
  mass_min < M < mass_max.
- This had been implemented before for f_salpeter and f_kroupa but was
  not changed at that time for f_chabrier