Update NeuroM definitions docs.
Update NeuroM definitions docs.

* No longer mention point-tree representation.
* Expand definitions, linking to NeuroM classes where applicable.

lidakanari/NeuroM,eleftherioszisis/NeuroM,liesbethvanherpe/NeuroM,BlueBrain/NeuroM,wizmer/NeuroM,juanchopanza/NeuroM,mgeplf/NeuroM