WIP: Offline Monitoring Naming Convention

Here, I am implementing the offline monitoring naming convention of
/run_XXXXXX/lb_YYY/... for single-lb histograms,
/run_XXXXXX/run/... for entire runs, and
/run_XXXXXX/lowStat_LBYYY-ZZZ/... for lowstat (20 lumi block) periods.

So far, I've only done the first of these three conventions by creating a new class called OfflineHistogramProvider, which I based on LumiblockHistogramProvider. Once the other two are implemented, I'll remove the work in progress.

Other details:

1. It does not make sense, I think, to keep the convention flag only at the histogram level, as this would essentially require the users to declare 'OFFLINE' for each of their possibly hundreds of histograms. Therefore, I have also created a convention flag for the GenericMonitoringTool. Well, only the python class, to be precise. Then, users can set this flag, and all of the histograms in that GMT automatically follow its convention.
2. Users can override individual histograms from following their parent GMT's convention by specifying convention=None in the call to defineHistogram()
3. I've made use of the existing kLBNHistoryDepth option, which defaults to 1LB when not explicitly (N.B. This default value is only set if the offline convention is used. No changes in behavior relating to other uses of kLBNHistoryDepth are introduced by this MR.)
4. I would assume a similar path convention is used Online? Therefore, I have left the possibility to implement this in AthMonitorCfgHelper.

Tagging the usual suspects: @pisarna @tbold @ponyisi

Chuck