Group LoKi functors by category

Similarly to #12 (closed): the list of LoKi functors can be organised by category, e.g.

• Reco-level
• MCParticle-level
• GenParticle-level
• Array functors (could be sub-categories of the above?)

Blocked by lhcb-dpa/wp6-analysis-preservation-and-open-data/lhcb-ntuple-wizard#37