Ojax

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.