Update Johnson-Cousin transformations

[kadrlica]

Douglas has derived some transformations from DES to Johnson-Cousins. We should update to use these to calculate M_V

https://des.ncsa.illinois.edu/releases/dr2/dr2-docs/dr2-transformations
https://cdcvs.fnal.gov/redmine/projects/descalibration/wiki/DESDR2_Tranformation_Interpolations#Johnson-Cousins-gt-DES

V = g{DES} - 0.465 (g-r){DES} - 0.020 [-0.5 < (g-r){DES} ≤ 0.2] (RMS: 0.012 mag)
V = g{DES} - 0.496 (g-r){DES} - 0.015 [ 0.2 < (g-r){DES} ≤ 0.7] (RMS: 0.011 mag)
V = g{DES} - 0.445 (g-r){DES} - 0.062 [ 0.7 < (g-r){DES} ≤ 1.8] (RMS: 0.024 mag)

Relevant code in ugali here

ugali/ugali/isochrone/model.py

Line 76 in 82abffc

def jester_mag_v(g_sdss, r_sdss):

and here:

ugali/ugali/isochrone/model.py

Line 329 in 82abffc

def absolute_magnitude(self, richness=1, steps=1e4):

Still need some general transformations for SDSS, DES, PS1, LSST.

[kadrlica]

See PR #97 too.