Add ATLAS_CHECK_THREAD_SAFETY to ElectronPhotonSelectorTools
requested to merge ATLAS-EGamma/athena:ElectronPhotonSelectors_ATLAS_CHECK_THREAD_SAFETY into master
Add the ATLAS_CHECK_THREAD_SAFETY check.
Bring in some of the changes from 21.2 (like factoring out the so called safeTH1.
Remove code that is no thread safe ans is not clean if it will be needed for release 22 Reco in that form