protections against missing objects

[madbaron]

This PR implements a couple of protections against missing collections and pointers in slimmed slcio files

[tmadlener]

Looks good to me.

At least for the ILD reconstruction we have started to try and eliminate all of the cases where missing collections might indicate an empty collection, and instead made all processors fill empty collections in case they haven't produced any output. The main reason is that a missing collection can otherwise mean missing because empty or it could also be a misconfiguration.

To make for an easier transition we have also implemented a PatchCollection processor, e.g. here: https://github.com/iLCSoft/ILDConfig/blob/fb10b66cdc2335d8f84443a14ec7fda64ab389ed/StandardConfig/production/ILDReconstruction.py#L322-L329 that can create missing empty collections on the fly if they should be present. The configuration is pairs of strings, following this description