Changes since v1.0

Change license to LGPLv3
Improve performance of navigations by keeping links between instances explicitly in memory
Swap order of phrases in navigations
Make referential attributes read-only
Support unrelate operation on associations formalized using identifiers (e.g. super/sub type associations)
Support navigations across associations classes without explicitly stating the association class name in the expression
Allow None values in in-memory queries (persisted null values still ignored due to ambiguities)
Allow links across instances with the same schema, but different meta model instantiations
Track positional information of SQL statements
Add experimental oal interpreter
Add the convenient functions xtuml.get_metaclass() and xtuml.get_metamodel()

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