Pointing Matrix Cleanup

[tskisner]

Refactor healpix and stokes weights operations:

• Move all current bundled healpix ops into the same source file as the PixelsHealpix code so that all device kernels are in the same compilation unit as the python wrappers.

• Add full unit test coverage for all bundled operations, comparing to the equivalent functions in healpy.

• Remove dead versions of the code from the deprecated libtoast internal library.

• Remove dead versions of the stokes weights code from the libtoast library and call the modern kernels in all unit tests.

• In the bundled ang to pix operations, perform range reduction to 0-2PI and also round phi angles to zero if they are close to machine precision from zero. This fixes the issue that was the original motivation for this PR.

• Update Stokes pointing weights to match latest document on polarization response.

• Remove stale versions of the stokes weights code and move all helper functions closer to the relevant operator kernel.

• Fix the use of the "cal" trait of the StokesWeights operator which was designed to be the name of the observation key to use for a detector calibration dictionary. This change propagated into the kernel code.

Additionally, add a small fix to the spt3g unit tests.

[tskisner]

Supercedes #701

[tskisner]

Since #701 will be deleted without merging, just wanted to attach the pointing weights document here for reference (the source notebook is in the new docs branch).

pointing_weights.pdf

[tskisner]

This PR is the restoration of #701, which was already approved. Merging now.