Add tests for section_tortuosity special case.
Add tests for section_tortuosity special case.

Looping section tortuosity is infinity. Add tests for that.

Resolves #520.

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