Draft: clearly marking IDs and indices in FTChannelID

Making clear in the software when a number is an ID (unique, logical identifier of an element e.g. 1 for station T1), a global index (unique, possibly illogical, starting at 0, sequential, e.g. 0 for station T1, or 13 for second quarter of the 4th layer), or a local index (not unique, but an index, e.g. 1 for the second quarter of the 4th layer).

Goes with Rec!2889 (closed) and Detector!197 (closed)

Detector#25 LHCBSCIFI-178 LHCBSCIFI-176