Pyxtuml

* generalize the `where_eq` argument to `select_many()` into an argument list of set operators that can filter and order query results.
* add the set operators `xtuml.order_by()` and `xtuml.reverse_order_by()`.

* 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()

* added python 3.6 to list of supported python dialects
* bumped development status to Production/Stable
* load: fixed deserialization of the string '
* load: allow integers to be persisted as GUIDs
* ooaofooa: populate unformalized associations correctly
* ooaofooa: display warning on unformalized associations
* ooaofooa: update schema to version 5.2
* examples: update ooaofooa schema gen
* meta: preserve order of attributes in indices

re-license to LGPLv3+