ConfDB2 File Handling, master branch (2019.12.13.)

Taught AtlasCMake how to handle .confdb2 type files, which were introduced in: gaudi/Gaudi!721 (merged).

Introduced a custom merge script (mergeConfdb2.py) for this based on Gaudi's merge_confdb2_parts script, since the script used by Gaudi was not directly usable by us. 😦 And writing a wrapper for the Gaudi script would've taken the same amount of lines of code, if not more.

At the same time re-worked the _merge_files(...) internal function a bit, so that Ninja builds would not have to re-run the merging commands on each build, even when it is not necessary.

Tagging @wlampl and @fwinkl.