Migrating STDM7 from 21.2 to master

This MR is migrating the STDM7 derivation from 21.2 to master.

This derivation has not been migrated previously. Hence, share/STDM7.py doesn't do what we want and shouldn't be compared. I figured there was no need to bother with the old-style configuration. That's why I only worked on the CA-style configuration.

@esoldato, @mdyndal, please have a look and let me know if you have comments. There are a couple of todo items left that I will discuss with the analysis team in the next days before removing Draft status.

... coming back to this after a while.

I started from PHYS and added some skimming. A few containers are removed, others are added.

In the long term we would also need some flags, specifically one that allows us to skim on the charged-particle multiplicity in MC, optionally. There was some discussion about flags in !57441 (merged). Can I adopt the approach? @jcatmore probably can advise.

Cheers, Philip