Remove many dependencies, introduce dependency from pyHealpix, various cleanups
This removes Nifty's dependencies from healpy, libsharp and many more and replaces them by a single dependency from the new pyHealpix package.
This branch also contains the changes from the 'strip_gfft' and 'reduce_cython' branches, so it is sufficient just to merge this one branch.
Tests have been adjusted accordingly and many cleanups were made; I especially removed obsolete (and confusing) bits of documentation.
Sorry for not producing several smaller merge requests ... but given the current turnaround time for patch reviews this would have become nearly impossible.