robust channel mapping for tracker

[edcallaghan]

PRs #1402 and #1476 introduced deserialization of tracker fragments, with a mapping from "minnesota number" panel labeling present in the data, to ``geographical coordinates,'' as used in Offline and as database keys, ultimately defined in fcl. As the tracker approaches a single, static, detector, this should be replaced with something more permanent (e.g., external text-file configuration as used by the calorimeter).

[eflumerf]

I think I've collected most of the potential interested parties in the "Assignees" list. We should schedule a meeting or similar to make sure everyone is content with the proposed solution before implementation.