[kimmerfx] Adding tests

[FelixKimmerle]

This PR adds:

• Unit and integration tests for the diffusion2d module using pytest
• A tox configuration for automated, isolated test runs
• A coverage report (HTML, printed to PDF; Functions tab)
• A requirements.txt listing the packages required for testing (used by tox
  in a virtual environment)

If you are on NixOS (or you use the Nix package manager on another Linux
distribution), you need to run:

nix-shell

in the root directory of the repository otherwise, tox may fail to install NumPy because libstdc++.so.6 cannot be found.

This uses shell.nix, which ensures that Python and tox are installed and that the library paths are set up correctly so libstdc++.so.6 can be found.