Add conditional requiredness for polarimetric data

This PR builds on PR 42 by adding a conditional requiredness that looks at whether the header is part of a polarimetric dataset.

The definitions for “what is a polarimetric dataset?” vary from instrument to instrument, so there are functions for each one where we can put that logic.

HELP NEEDED

Right now I only made 3 keys polarimetric_required: POL_NOIS, POL_SENS, and STOKES (although I think the later should maybe just be required?). There might be more, but I don’t know where to get that information. Advice welcome.