Improve MCHierarchy functor collection

Hello @amathad, I noticed that the BkgCat int is in fact not in the MCHierarchy functor collection, and I think it should really be there by default. I would suggest the following:

• Add BkgCat by default and maybe add a flag store_bkgcat = True.
• I would remove the storing of keys by default as I don't think the bulk of analysts need those for anything. I presume that's only useful for special/extended studies/DSTs. Add a flag store_keys = False. And add the key related directly to the particle, which is not present at the moment in the collection.

Let me know your thoughts, thanks.