Document what point, segment, section, neurite are.

[asanin-epfl]

Either to move those definitions from NeuroM or duplicate them in MorphIO as well. What is more preferable? @tomdele @mgeplf ?

The former seems better. Because NeuroM relies on MorphIO, it would be better to describe in MorphIO, and only refer to it in NeuroM docs.

[tomdele]

Guess having them in morphio is mandatory.
We can use a link pointing to the morphio doc in NeuroM since they have the same definitions (NeuroM using morphio internally anyway).