Simplify neurite type calculation for section trees.
Simplify neurite type calculation for section trees.

* Each section can have only one type. Set the tree node types
  from the section info. The type of a neurite is taken from the
  type of its first tree node.

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