add PadAreaBoundary enum to remove magic numbers from BaseTpcGeo

Pretty much cosmetic fix, related to proper naming.
BaseTpcGeo is base invariant class, and has to be compact & as readable as it gets for any developer.

DO NOT REMOVE THIS BOXED MESSAGE

Code in this merge request is subject to review by Code Owners.
The list of reviewers is posted in the comments section.
After all of the code is approved, please put a brief notice about it at the beginning of this message.

NOTE: Merging this code without getting it approved by Code Owners will result in suspending your developer rights!!!