Clarify units of SP properties
Clarify units of SP properties

Changed names of properties in `StellarPopulation` to clarify their
units. Also added basic docstrings. Masses are linear in solar masses,
star formation rate is linear solar masses per year, while luminosity
and age are logarithmic.

bd-j/python-fsps,jrleja/python-fsps,nell-byler/python-fsps,dfm/python-fsps,ajmendez/python-fsps