Implement remaining healpy compatibility functions

[astrofrog]

We can still add compatibility functions (just thin wrappers) for the following:

• healpy.boundaries (using healpix_boundaries_lonlat)
• healpy.nest2ring (using nested_to_ring)
• healpy.ring2nest (using ring_to_nested)
• healpy.reorder (using nested_to_ring or ring_to_nested)
• get_interp_val (using interpolate_bilinear_lonlat)

[astrofrog]

I don't think we want to implement query_disk since we use a different algorithm that seems to give more reliable results so no point trying to reproduce the healpy algorithm.

[cdeil]

Here's what we have now:
http://astropy-healpix.readthedocs.io/en/latest/api.html#module-astropy_healpix.healpy

As mentioned, reorder is still missing:
http://healpy.readthedocs.io/en/latest/generated/healpy.pixelfunc.reorder.html

@astrofrog - Do you think we should add it now, or wait for users to request it?
@lpsinger - Do you use it? Or are there others that are missing that we should add before cutting a new release?

[astrofrog]

@cdeil - reorder should be easy enough to implement by using nested_to_ring and ring_to_nested. But no rush if no one is asking for it.

[lpsinger]

Do you use it?

Yes I do.

Or are there others that are missing that we should add before cutting a new release?

Not sure at the moment; it would take me an hour or two to update astropy-healpix branches of my code and test them.