Moved bestfit Salpeter alpha calculation to module

-my_em.py: contains module maximimze_em_salpeter(...)
 for a given set of input alpha values, an isochrone, a photometric
 dataset, photometric system, and distance modulus, calculates EM
 likelihoods for each alpha value.
 calculates minimum alpha and formal errors from delta L + 1/2.

-main.py: calls in maximize_em_salpeter module, tested that results
 are equivalent. This cleans up the clutter in main.py

-next commit: create version of main.py used for EDA with different
 Nstars, mass ranges, error sizes, etc.