rename / redefine the parameters for the healpix index

[keewis]

We currently use rot_latlon, nside, and nest as grid parameters for healpix (mostly because those were inspired by healpy). However, I'd like to propose to a few changes:

• nside: nside describes the number of subdivisions per side, and is always a power of two. That means that by taking the base-2 logarithm of nside we get a new parameter with a definition that is much closer to the resolution of other DGGS. New names could be order (from healpy), depth (from cdshealpix), level, or indeed resolution.
• nest: In healpy, this is used to switch between the nested and ring indexing schemes. However, those are not the only ones in use, the unique scheme also exists. As such, a boolean parameter does not seem to be a good idea. Instead, I'd use a string: indexing_scheme="nested", indexing_scheme="ring", or indexing_scheme="unique".
• rot_latlon: I would call this just rotation (proj calls this rot_xy to be consistent with the projections), where just like with healpy / cdshealpix the order would always be longitude then latitude. The values would still be specified as a list / 1D array. For example: rotation=[0, 30].

What do you think?

[benbovy]

Those changes all make sense to me.

#35 would make it easier supporting both nside and resolution as inputs and validate the value.