l2_afp: L2 Alternate "Full-Physics"

This package implements "alternate" or "augmented" variations of the OCO-2 "Full-Physics" forward model and the associated L2 Retrieval.

This is a work in progress, so do not expect the code to be stable in any way.

This requires the RtRetrievalFramework (also know as the "Full-Physics" forward model) from NASA JPL to be installed beforehand, with the python SWIG wrappers included in the build. See https://github.com/nasa/RtRetrievalFramework