HIMonitoring: enable thread-checker and package cleanup

Frank Winklmeierrequested to merge
fwinkl/athena:himon_thrchk into 22.0

Do not modifiy the global histogram style via gStyle. This is the unused legacy monitoring code, so this should have no impact.

Also move private headers to src/ directory and only build component library.

Targeting 22.0 since monitoring developments will be done there.

Merge request reports