Update getspec call in get_isochrone subroutine

For consistency with FSPS r119.
Update filter list for r119

FSPS r119 has changed the filter order, not simply appended new filters
to the list.
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.
Merge pull request #6 from dfm/fix_r119

Fixes for FSPS r119