Likelihood = func(alpha) with various sample cuts

- Defined df.dist_center series = distance from center of dsph.
- Make cut in distance (n_half*rhalf_dwarf) & magnitude
  (magmin < mag < magmax), no color cut but can be implemented
  easily.
- Re-indexed phot DF so elements (rows) can be called in a loop
  starting from i = 0 (explained in blog).
- Create set of three plots to guide in selection of region cuts
  (see 2nd point above)
- Made CMD plot overlaying stars in cut sample, as well as the
  isochrone used in calculation of ln L.