Testing my LF calculation (test_lf.py)

Description of changes:
======================
- Added: read_lf module.
- dmass_arr = > abs(dmass_arr) in dN/dM * dM calculation.
- Compared successfully my r-band LF given a Dartmouth isochrone
  compared to the LF computed by the Dartmouth code for said
  isochrone.
- Note: LF calculated by Dartmouth code includes non-MSTO, I
  excise those portions of the rmag array as we are not using
  non-MS stages for fitting, and checked that our MS LFs match.

To do:
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- There is still a small offset in r-band (seems constant).
  This may be because I am choosing to use the rmag points in
  the isochrones as left-edges of their respective bins. To
  do more properly, need to have dMass bins between r +/- dr/2.
- Mass - magnitude relation must be monotonic, need to
  cut off isochrone at or around MSTO (necessary?)

 [Originally had failed to match Dartmouth LF as the LF I
  requested from Dartmouth had x = 2.35 instead of x = -2.35]