f_salpeter: change of min_mass and mass_max

The dN/dM function is no longer truncated for M
outside of M_min and M_max, only outside of 0.1Msun
and 100 Msun.

M_min and M_max are instead *normalized* using
only the integral of dN/dM * deltaM between
M_min and M_max. The normalization is then
applied to the entire range of masses in the
isochrone.

The rationale is that cuts should only be
done on the data, and not on the isochrone.
main.py: Current Status

- For Salpeter alpha >~ 2, I can recover the best-fit values in
  the limit of infinitesimal errors, <~ 0.0001 dex. Thus the
  normalization done in the previous commit as well as the
  EM formula used is adequate. This is also for large N>10^4
  and magnitudes down to ~ 30.

- For larger errors, or lower
  alpha values, cannot recover the fits. In particular, for
  lower alpha, alpha_fit tends to be higher than alpha_in.

- Next: exploring whether smoothing isochrone in mass and/or
  magnitude will improve the fits.