Tokamesh

- Fixed an issue in `BarycentricGeometryMatrix` where mesh edges with a direction vector whose z-component is comparable to the float precision, but non-zero (i.e. almost exactly horizontal edges) could cause incorrect intersection calculations.

- The `Triangle` source code and the cython wrapper used to access it, which was previously bundled with `tokamesh`, has been removed in favor of using `triangle` PyPI package, which has been added as a project dependency.

- Increased minimum required Python version to 3.9.
- Moved all package metadata to the `pyproject.toml` and deleted the `setup.cfg`.
- Updated `ReadTheDocs` config files so they meet current requirements.
- Refactoring to improve module structure, resulting in new `mesh` and `utilities` modules.
- Improvements to type-hinting and documentation.

- ZedThree has developed a Cython interface to the `triangle.c` code such that it can be used directly from Python, instead of communicating inputs / outputs via writing / read files to disk. Additionally, `triangle.c` is now compiled for various combinations of operating systems and Python versions when a Tokamesh version is published, such that it no longer needs to be compiled at runtime on a user's machine.
- [Black](https://github.com/psf/black) has been applied to the entire code-base, and is now also automatically re-applied following commits / merges via GitHub actions.

- Added the `find_triangle` method to `TriangularMesh` to enable lookup of which triangles contain a given set of points.
- `TriangularMesh` and its methods now check the validity of their arguments.
- `TriangularMesh.interpolate` can now handle multi-dimensional numpy arrays as inputs.