Changes since version 1.0

Update to pyxtuml version 2, which means:
- 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
Use short circuit evaluation of expressions containing 'or' and 'and' operators
Allow unique_id type parameters
Allow specification of key letters within angle brackets for applicable parameter types. Example
Add support for "relate/unrelate using" syntax, inspired by OAL syntax. Example
Add support for modulus operator (%)
Add support for the caret operator (^)
Cast left-hand operand to a set if the right-hand operand is a set
When a fragment is printed, display its key/values
Improve type checking of instance and instance set parameters
Drop comments during tokenization

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