Transfer CREFL rayleigh corrections from satpy to pyspectral

[djhoese]

At the benefit of development/testing time the original python versions of the crefl algorithms for rayleigh correction were added as modifiers to satpy. These should really be in pyspectral. There is one algorithm for VIIRS/MODIS and one for ABI.

https://github.com/pytroll/satpy/blob/master/satpy/composites/crefl_utils.py