Categories of PIDs

[UlrikeS91]

This was raised in PR #479, in this comment:
"to 3) up to @apdavison ; from my side of course can this be added to whatever we see fit. Question: should we not rather introduce categories of PIDs for the different RPs/RPVs to more fluently add new PIDs when needed? @olinux would that be dynamically adopted in the KG system?"

I'm adding it as an issue to keep it open since I am about to merge the PR and this is the only open discussion point. Whether or not the ID from the PR is also added to model versions, can be discussed at a later point. If we conclude to add those categories, it can be fixed in parallel with the update.

[lzehl]

@openMetadataInitiative/openminds-developers I though about this a bit more and I'm not sure if it is worth introducing different categories of PIDs for the different research products. I would suggest we have one category for PIDs and allow them for all RPs/RPVs as selection (even if they are not used by all of them)