Ensure compatibility with DeepXDE and wrap operators implementations

[samuelburbulla]

• [x] Add test that ensures that functionality from 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.