TrigGenericMonitoringTool: copy defineHistogram from AthenaMonitoring

Frank Winklmeierrequested to merge
fwinkl/athena:trigmontool_legacy into master

TrigGenericMonitoringTool is deprecated and is only kept in master as long as we need to keep running the Run-2 trigger. As such it is better that it contains its own copy of defineHistogram. That way we can freely modify the new version in AthenaMonitoring and don't have to worry about backwards compatibility.

cc @tbold

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