AsgTools Cherry-Pick, 21.2 branch (2019.02.01.)

This is a cherry-pick of the changes I made to AsgTools with !20404 (merged) in the master branch. @krumnack's nice cherry-pick (!20786 (merged)) reminded me that I promised to do this...

The updates are all just ROOTCORE -> XAOD_STANDALONE changes, and adding copyright messages to the modified files.

Nils, the copyright messages were a bit different in 21.2 and master on the touched files. In 21.2 you've put a Iowa copyright on the files, while in master these were removed. Since with !20786 (merged) you didn't touch these (you may not have noticed this, but maybe you did), I decided to update the 21.2 files to be in sync with master in this regard.