Calorimeter object needs documentation

[brownd1978]

The Calorimeter geometry object obtained via GeomHandle has an opaque interface and lacks documentation. For instance CalorimeterGeom/inc/CaloInfo.hh provides a text-string keyed interface, but there is no documentation what the valid text strings are. Documentation should be provided in the class headers, and in the appropriate section of the mu2e wiki, which is currently completely blank. That should also describe the different geometry systems used in the calorimeter (disk, crystal, etc) and their relationship to each other.