read_iso_darth:

- Created a small grid of isochrones with the following params:
  * [Fe/H] = [-2.5,-2.25,...,-0.75]
  * [a/Fe] = [0.0,0.2,0.4]
  *  age   = [6,8,10,12,14] Gyr
  The grid was created using make_isochrones.py located in a separate
  directory. That program is a wrapper for isolf_split.f and iso_interp_feh.f
  provided by Dartmouth isochrone team.
- Updated read_iso_darth(age,feh,afe,system), previously read_iso():
  Takes four parameters: age, feh, afe, system, where
  system = 'wfpc2' to read from isochrone library in WFPC2 system.
- Note: read_iso_darth(...) changes magnitudes of F606W and F814W
  bands from VegaMAG to STMAG using zero-point differences found in
  synphot table in HST help pages (see relevant blog post from 09/02/2014