Read-in HST data & update color/mag labels

- Updated read_phot module to read-in HST photometry data, for now
  in the format of the Hercules dataset file provided by MG.
- Modified names of columns in photometry dataFrames to make them
  generic, ie, likelihood will be calculated on columns "color"
  and "mag".
- Moved cuts in color/mag for both CFHT and HST datasets to within
  read_phot, adding a keyword kwarg: cuts = True/False. If True,
  cuts will be made following cuts provided by MG (HST data) and
  a combination of projected separation and magnitude (CFHT data).