ACS_606_814_to_VRI transformation

Uses the transformations presented by Sirianni 2005 Table 22..
I present the math derivation for a Sage notebook.
Implement VRI_to_gri

Transformation is based on Lupton 2005,
http://www.sdss3.org/dr8/algorithms/sdssUBVRITransform.php#Lupton2005
ACS_475_to_BVRI and BVRI_to_ugri

These transformations are used for PHAT ACS data sets to convert
F475W-F814W photometry to SDSS ugri photometry.