Ensure compatibility with DeepXDE and wrap operators implementations

- [x] Add test that ensures that functionality from [DeepXDE](https://github.com/lululxvi/deepxde) (torch backend) can be used with continuiti:
- geometry
- physics-informed loss
- rebuilding pi-deeponet with continuity
Added in #133
- [ ] Add a wrapper that allows to use DeepXDE PDE implementations (including geometry and bc) to be used in a physics-informed loss function in continuiti
- [ ] Investigate if continuiti is faster than DeepXDE
- [ ] Wrap operator implementations in a `continuiti.wrapper.deepxde` module.