Add the ability to change the NSIDE of a HealPix object

[AdamGoldstein-USRA]

Propose to add an object method HealPix.to_nside(nside), that will change the resolution of an existing HealPix object and return a new HealPix object with the requested nside. This would be called in the following way:

# convert a NSIDE=64 HealPix object to NSIDE=128
hpx128 = hpx64.to_nside(128)