Add a test on unbranched, non overlapping segmentGroups

[pgleeson]

e.g. see
https://bitbucket.org/mbezaire/nc_superdeep/src/fab4d82bae57a42a7da0e595d0cf04f203787295/neuroConstruct/generatedNeuroML2/pvbasketcell.cell.nml?at=default

segmentGroups marked with neuroLexId sao864921383, e.g.

<segmentGroup id="bcdendBAS2_2" neuroLexId="sao864921383">
<member segment="39"/>
<member segment="40"/>
</segmentGroup>

can be used to specify a subset of segmentGroups in a cell which contain unbranched (each segment has a unique parent in this group apart from the first) and non overlapping (every segment in cell belongs to one and only one of these segmentGroups) groups.

A test for this should be added to https://github.com/NeuroML/org.neuroml.model/blob/development/src/main/java/org/neuroml/model/util/NeuroML2Validator.java

[pgleeson]

Note, using this annotation is not manditory, it just faciliates mapping the morphology efficiently to NEURON, see:

https://github.com/NeuroML/org.neuroml.export/blob/development/src/main/java/org/neuroml/export/base/JSONCellSerializer.java
https://github.com/NeuroML/org.neuroml.export/blob/development/src/main/java/org/neuroml/export/neuron/NeuronWriter.java