Remove references to geom files in the code

Back in the day, we used to specify core layout in what was called a facemap or geom file. While we retain the code that supports this input mechanism (see https://github.com/terrapower/armi/blob/master/armi/reactor/systemLayoutInput.py), it is mostly there to enable the migration performed here: https://github.com/terrapower/armi/blob/190c0caccfd36a0442118a1ae666c2a20c52a4d1/armi/reactor/blueprints/__init__.py#L524

Instead we recommend using grid blueprints instead. However, when these were completely separate input files, they became part of some major ARMI function signatures where input stuff gets passed around, usually as geom. We should try to remove as many of these as we can.

Actually searching around, I’m not seeing as many as I remember (go us!), but a glaring example is in the Case class: https://github.com/terrapower/armi/blob/190c0caccfd36a0442118a1ae666c2a20c52a4d1/armi/cases/case.py#L73

[keckler]

In fact, I would love it if XML support was dropped.

[john-science]

Okay, we definitely support basic RZTheta geometries in YAML format.

Nick believes there are some extra pieces of information in the RZTheta geom files that isn’t supported in RZTheta YAML files. So I need to find the specifics of what we don’t support, and build support for it.