Arango-orm

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- Proper handling of `_rev` values. Adding or updating documents loads/reloads `key_` and `rev_` from the db
to keep the collection object consistent with backend. Without it subsequent updates on the same object would
fail due to `_rev` mismatch.
- Bugfix for `relationship` function (so it uses `key_` instead of `_key`)

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- Support for providing filter conditions for Graph.expand

python
graph.expand(condition="vertex.city == 'Gotham'")

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- Conversion from marshmallow to pydantic 2.x (Backward incompatible)
- Use pydantic style model definitions. Model classes still inherit from `Collection` or
`Relation` classes but these classes are now derived from pydantic's `BaseModel` class and
provide some additional functionality related to ORM.
- `Collection._dump` and `Collection._load` methods are removed in favor of already present
pydantic methods for the same tasks. Use `Collection.model_dump` and `Collection(**kwargs)`
for dumping and loading data from models respectively.
- Due to how pydantic works, built-in collection/relation fields `_key`, `_from` and `_to` must
be defined as `key_`, `from_` and `to_` in models. However `property` getters and setters are
present to allow accessing and setting these using the built-in names. See examples for deatils.

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- Add Collection.get_schema_class(), schema() return instance instead
- Fix Collection.schema(). Marshmallow schema is recreated in case of class inheritation.
- Added Conditional Checking Of Marshmallow Fields

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- Documentation updates and dedicated documentation site at: https://arango-orm.readthedocs.io/en/latest/index.html
- Graph.delete_tree method to delete an already "expanded" object and all it's linked data.
- Fixes memory leak related to marshmallow schemas not getting cleaned up correctly. Thanks SoundsSerious

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Merging PR from chris-green with thanks.

- Return embedded objects in schemas (using Nested fields) as their object equivalent instead of dicts.
This change instanciates all objects upon load using the post_load decorator upon schemas,
automatically instanciating both top level objects and their embedded counterparts.

- Add docker-compose and pytest for quick, consistent setup of application and tests for contributors to package.

- Added getting-started to explain how to get started on how to run tests for package.