Check whether homemade rhealpix and healpix can be replaced by proj versions.

[rggibb]

I'll leave projection_wrapper-py, and the pj_healpix and pj_rhealpix homemade projections where they are, and raise an issue to flag that we need to check whether the versions of healpix and rhealpix in proj.4 (now proj.7) can be used. At the time the code was last checked by Alex Raichev, the code in proj wasnt yet working, so the homemade version here had to be used.

Originally posted by @rggibb in #2 (comment)

[ndemaio]

This is now in progress, with fairly...explosive...results so far. We'll keep picking at it until we know more.