Add ability to create a PixelDistribution from a sky footprint.

[tskisner]

In most workflows, a PixelDistribution is created by passing through the detector pointing on each process and determining the locally hit pixels and submaps for a given pixelization scheme. This work adds support for creating a distribution from a pre-existing sky footprint. Each process will have the same hit submaps in this case. This is a stepping stone towards building the RHS out of core while keeping the locally hit pixels in node shared memory.

[tskisner]

Note: this PR also re-enables a couple tests that were accidentally disabled during tests.

[keskitalo]

Looks good. Do I understand correctly, that we have no submap support for rectangular pixelization?

[tskisner]

Re: submaps:

• The WCS projections in general support submaps, but this domain decomposition is just over the pixel index (so blocks of rows / columns in the projection). In practice this is not as useful as nested healpix submaps, since a typical scan strategy from the ground means that all processes essentially cross all submaps.
• One could consider supporting a tiled scheme like pixell, and treat each tile as a submap. I don't see much technical benefit over healpix, but it is something we could support for compatibility reasons.
• For the purposes of this PR and the footprint distribution helper function, a WCS projection is just assigned one submap (for the reasons above).

I'll work on the other fixes shortly.