Switch from healpy to astropy_healpix for trivial functions
This is part of a gradual transition from healpy to astropy_healpix. This patch migrates the following function calls (the easy ones):
-
hp.order2nside
->ah.level_to_nside
-
hp.nside2order
->ah.nside_to_level
-
hp.nside2npix
->ah.nside_to_npix
-
hp.npix2nside
->ah.npix_to_nside