Update default StokesWeights operator

[tskisner]

When generating the elements of the pointing matrix. The mathematical basis for these changes can be found in this document:

HWP_algebra.pdf

One important note is that after this PR is merged, all instrument models for workflows that use a HWP will need to ensure that a focalplane table column exists for the detector gamma angle.

• Update sign convention to match accompanying document (to be added to documentation) given our recent formalization of the focalplane coordinate frame.

• When using a HWP, require per-detector offsets in the focalplane table describing the fixed angle from focalplane frame X-axis to detector frame X-axis.

• Correctly handle the cases of fixed HWP angle versus no HWP.

• Add unit tests that cover several permutations of I/Q/U sky pixel values, fixed HWP angles, and detector orientations.

[tskisner]

Ok, this is finally ready for review.