Reworking the NeuroML export.
Needed to export in micrometers rather than nanometers,
and to export a list of skeletons each as a NeuroML Cell.
Synapses are still not defined, and there is no entry in urls.py
or access from the web client yet.
Merge branch 'master' into neuroml
More fixes for NeuroML export. Exports the XML but correctness has not been validated yet.
Fixes for exportneuroml.py

The Segment vs SegmentParent objects in NeuroML python are not entirely clear to me.
Merge branch 'master' into neuroml
Merge branch 'master' into neuroml
Export for NeuroML Level 3, version 1.8.1 (validated).