HWP angle loading improvements

@mhasself I think this needs to be a small context hook or something but:

We need to work out how we want to get the solutions calculated by make-hwp-solutions into the top level of the AxisManger. I have the skeleton of make-hwp-solutions now running on the site system and I set up the context so the results get loaded into aman.hwp_solution. That sub axismanager includes hwp_angle_ver1 and hwp_angle_ver2. We need a way to pick one and to load that into aman.hwp_angle so it can be used in later code.

I think this is some post load hook that is configured to "take sub field X and move to top.Y" but I haven't thought more than that. I do have other examples (tod processing results) where I would like to take other loaded fields and move them around to new fields, so a solution more general than for hwp angles would be nice.

Start by tracing the context loading that populates aman.hwp_solution, including hwp_angle_ver1 and hwp_angle_ver2, and review the mentioned TOD processing examples. Define how a selected subfield becomes aman.hwp_angle and how the same post-load approach could move other loaded fields; done means the chosen angle is available at the top level and the general behavior is documented or tested.