Group specifications in cross section control setting are weird

[keckler]

A typical example of using the crossSectionControl setting might look something like this:

  crossSectionControl:
    GA:
      geometry: 0D
      fluxFileLocation: \\some\location
    LA:
      geometry: 1D cylinder
      validBlockTypes:
        - control
      blockRepresentation: ComponentAverage1DCylinder
      driverID: AA
      numExternalRings: 2
      mergeIntoClad:
        - gap
      meshSubdivisionsPerCm: 1.0
      xsMaxAtomNumber: 10
      minDriverDensity: 1e-8
      xsPriority: 1

Is it odd that XS groups in this setting are referred to by their type+burnup group (i.e. GA as compared to just G)? Here are a few potential points of confusion:

• The user might look through their blueprints and see that XS type GA is not defined for any blocks, for instance. This is confusing.
• This would seem to preclude 2-digit groups (see Document the possibility of 2-letter XS types #1709 ) from being correctly handled.
• It might make one think that the setting is referring to only the first burnup group of a given XS type, in which case the later burnup groups of that type would fall back to the default.
• Is the driverID: AA referring to the first burnup group of XS type A, or generically all of group A, or the 2-digit group AA? Is this driver group updated with burnup somehow?

It doesn't help that the crossSectionControl setting is hardly mentioned in the user docs.

I would suggest clarifying these few points, primarily by adding thorough documentation of the crossSectionControl setting, but also potentially by updating the way XS groups are specified within the setting itself.