CAL-TKR data structures

We've had a discussion between the CAL and TKR groups, and have a couple of proposals that should make the code a bit clearer:

• We'd like to reseve the term "layer" to mean either a single layer of silicon (either x or y) or a single layer of CAL logs.  An x-y pair of layers in the TKR will be called a "bilayer" or an "xyLayer."  We still haven't come up with a way to distinguish the "backwards" numbering in TkrRecon.
• We'd like to define a single global enum axis {X, Y, NONE} to replace the 4 or 5 separate ones that are currently in the code, say in GlastAxis.  Then TKR will define a Glastaxis::axis view, and CAL will define a GlastAxis::axis orientation.