Add support for the string filling of histograms
requested to merge tbold/athena:generic-monitoring-tool-supports-strings-histogram-fills into master
This MR adds feature to the GenericMonitoringTool that allows to have Scalars and simple Collections (e.g. vector) to contain strings. In such case the ROOT TH1 Fill API using strings is used. Only 1D, unweighted histograms are supported. Exception is thrown if weighted filling is requested. Maybe I shodl als add exception in case of 2D and other types of filling are used? Tagging @fwinkl @ponyisi @cburton Addresses: ATLASDQ-727