Ojax

Add new `Alien` field type to denote "incompatible" fields that should crash
PyTree flatten / unflatten operations:

- New features:
- Added the `ojax.Alien` Field class.
- Added `ojax.alien` for alien field type declaration.
- Added `ojax.AlienException` which will be raised when flattening /
unflattening PyTrees with alien fields.
- Other changes:
- fixed wrong `NoAnnotationWarning` warnings when a `property` is defined in
`ojax.OTree`.
- Updated the documentation to include the new alien field type.
- Added test coverage analysis and improved unit tests

Rework `ojax.OTree` field type representation.

Using custom subclass of
`dataclasses.Field` instead of setting custom metadata. This simplifies field
type checking and allows for further subclassing. Specifically:

- Classes `Aux`, `Child`, `Ignore` are added which indicates the type of
`OTree` field. They all inherit from the abstract base class `OTreeField`
which is itself a subclass of `dataclasses.Field`.
- The function `ojax.get_field_type` and the enum class `ojax.FieldType` is
removed since they are now unnecessary.

Note that users should still use functions `aux()`, `child()`, `ignore()` to
declare field types since directly using classes could generate confusion for
type checking. This is also in line with the standard `dataclasses` usage.

Fixes unnecessary missing annotation warning for non-user-defined attributes.

Change doc theme. Improve documentation.

- Other changes
- Now uses the sphinx_book_theme for Sphinx documentation.
- Finetune documentation.

Rework API. Add unit tests.

- Breaking changes
- Renamed `ojax.OTree.infer_field_type()` to
`ojax.OTree.__infer_otree_field_type__()`.
- New features
- Added the `ojax.fields()` function to get the fields from an OTree. It can
also return fields of a certain field type.
- Other changes
- Renamed `test/` folder to `tests/`.
- Added unit tests.
- Added CHANGELOG.md (this file).

First version.