Rewrite crefl rayleigh algorithms with cython

[djhoese]

After #51 the crefl algorithms should be rewritten in cython for the best performance. Currently they use numpy/dask arrays, but these are wasteful with intermediate/temporary arrays. Doing things by pixel should theoretically perform better and reduce the memory usage.

@ralphk11 and @wroberts4 have the most experience with these algorithms probably. @mraspaud also wrote the original viirs/modis crefl python code that is in satpy although @ralphk11 had one in the past too.