Cherry pick fixes
This is a collection of fixes I've accumulated over the last few weeks:
- Bump release to 21.2.120
- Allow the track writer to get variables directly from the DL2 class in Athena. I hope this makes it easier to do things consistently in the training and inference code.
- Configure DL2 in the single b-tag dumper with a more general structure. This allows us to (for example) change the names of the input and output variables for DL2.
- Move to using
nlohmann/json
do dump metadata in thedump-hbb
code. - Skip input files that can't be accessed by xrootd in the hbb dumping code. We record this in the metadata so it's not silent, but having the job fail in the grid is annoying.
- Bump the cmake implementation. This is strictly needed for anything, it's just good practice to keep this number up to date.