Alchy

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- Convert null relationships to ``{}`` when calling ``Model.to_dict()`` instead of leaving as ``None``.

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- During ``Model.update()`` when setting a list relationship automatically instantiate ``dict`` values using the relationship model class.

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- Allow ``alias`` keyword argument to ``Query.join_eager()`` and ``Query.outerjoin_eager()`` to be a ``dict`` mapping aliases to join keys. Enables nested aliases.

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- Fix handling of nested ``Model.update()`` calls to relationship attributes so that setting relationship to empty ``dict`` will propagate ``None`` to relationship attribute value correctly.

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- Add ``query.LoadOption`` to support nesting load options when calling the ``query.Query`` load methods: ``join_eager``, ``outerjoin_eager``, ``joinedload``, ``immediateload``, ``lazyload``, ``noload``, and ``subqueryload``.

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- Replace usage of ``classproperty`` decorators in ``ModelBase`` with ``classmethod``. Any previously defined class properties now require method access. Affected attributes are: ``session``, ``primary_key``, ``primary_keys``, ``primary_attrs``, ``attrs``, ``descriptors``, ``relationships``, ``column_attrs``, and ``columns``. (**breaking change**)
- Proxy ``getitem`` and ``setitem`` access to ``getattr`` and ``setattr`` in ``ModelBase``. Allows models to be accessed like dictionaries.
- Make ``alchy.events`` decorators class based.
- Require ``alchy.events`` decorators to be instantiated using a function call (e.g. ``events.before_update()`` instead of ``events.before_update``). (**breaking change**)
- Add ``alchy.search`` comparators, ``eqenum`` and ``noteqenum``, for comparing ``DeclarativeEnum`` types.