Rework Pixtable's self-calibration

Not completely ready, needs a fix for Python 3 (done) and some docs improvements (done).

The new algorithm is described in the docstring: https://git-cral.univ-lyon1.fr/MUSE/mpdaf/blob/77d23db4c2310b8b29cd6d88aa59f3074fa3f88e/lib/mpdaf/drs/pixtable.py#L1743

And given the number of changes, it is easier to look directly at the new version of subtract_slice_median.c: https://git-cral.univ-lyon1.fr/MUSE/mpdaf/blob/77d23db4c2310b8b29cd6d88aa59f3074fa3f88e/src/subtract_slice_median.c

@laure.piqueras - If you want and have some time to take a look, would be much appreciated !

Also cc @peter.weilbacher and @roland.bacon , if you want to have a look.