Updates for use of observing sessions

[tskisner]

This has changes needed to support the use of the existing observing sessions across the code base. It also fixes several bugs discovered in that process.

• SimGround operator now has a trait specifying the input focalplane key to use for splitting telescopes into observations within one session. Move some code out of exec() and into a separate method.

• In atmosphere simulations, group detectors by frequency range and separately calculate the common absorption and loading for each.

• Use session UID instead of observation UID across operators where that was the intended behavior.

• Support multifrequency fake focalplanes for unit testing.

• When simulating noise models whose streams are detector PSDs, use the detector UID for the index.

• Fix typo in Bandpass.get_range() method.

• Fix bug introduced recently in Poly2D filter.

• Fix unit test for YieldCut operator to account for case of running with MPI where each process has a subset of the global detectors.

[tskisner]

Note that I am still working on resolving an MPI unit test failure on the Poly2D filter.

[keskitalo]

This PR should be rebased to resolve any potential conflicts from the processing mask merge.