Implement less naive path lengths calculation.
Implement less naive path lengths calculation.

When the path lengths of all the sections in a tree
need to be calculated, we can cache the individual
section lengths in one pass.

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