Add IMF and PMF curves obtention

[Gabriel-p]

After #484, use the masses to derive the IMF and PMF of the cluster.

Keep in mind when moving stars over the isochrone to obtain their masses:

• Present results using: all stars in cluster region (with and without MPs affecting each star's mass), only those stars with MP>0.5, only those stars that passed the membership filter (i.e.: the local density removal), etc.
• Use stars located far away from the isochrone or reject them?
• Use several MP cuts?
• Summing all masses gives the SC's mass.

Read:

• CCD photometric and mass function study of nine young Large Magellanic Cloud star clusters, Kumar et al. (2008)
• On the function describing the stellar initial mass function, Maschberger (2012)
• Mass distribution and structural parameters of Small Magellanic Cloud star clusters, Maia et al. (2013).
• The Range of Variation of the Mass of the Most Massive Star in Stellar Clusters Derived from 35 Million Monte Carlo Simulations, Popescu & Hanson (2014)

[Gabriel-p]

This is very complicated (almost impossible) to generalize to every particular case. Hence, I've moved this to its own package here: https://github.com/asteca/IMF