Allow interactive plotting to GUI.
Allow interactive plotting to GUI.

Use DISPLAY environment variable to determine whether to
use non-GUI 'Agg' backend or not. This means systems that
do not use DISPLAY don't get interactive plotting functionality.
That's just too bad.

Change-Id: Iea331229e7d7c6d63d25705867b59cb9fd6614cc

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