unstable documentation:
links towards reference texts describing how gem-graph operates
(and why all the parsed items are useful, compulsory or optional) missing.
links towards description of the gem-graph model structure (and its XML implementation) missing.
Warning about the word 'node': possible confusion between:
- the xml tree 'nodes' used in the model xml file and
- the gem-graph 'nodes' that are local space units used in the geometric space representation