This PR has the implementation of flat relative optimal extraction method.
the implementation is included in modules/spectral_extraction/src/alg.py (flat_relative_optimal_extraction) and spectral_extraction (for the primitive by setting extraction_method to be 'fox' in the recipe)
the implementation is mainly based on "Flat-relative optimal extraction A quick and efficient algorithm for stabilised spectrographs" by M. Zechmeister etc.
This PR has the implementation of flat relative optimal extraction method.