AthenaMonitoringKernel: Update doxygen documentation

Frank Winklmeierrequested to merge
fwinkl/athena:athmon_docs into master

Major cleanup/extension of doxygen documentation and examples:

  • Added a MonAPI group page (shows up under Modules) that summarizes the user-relevant API
  • Add unit test and examples for filling with weights
  • Change some variable names in the interface (e.g. converterToDouble -> accessor)

There are no functional code changes in this MR. cc @tbold @ponyisi @cburton

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