Unit Test boilerplate

[pierre-wilmot]

Adding the boilerplate for unit test setup with some minimal grid tests.
Hoping to add test for indexing / hashing in the future.

[Tom94]

Thank you very much for the PR!

catch.hpp seems to be an external dependency -- could you move it into its own dependencies/catch subfolder in order to make code statistics & licensing work out better? If it comes from a public github repo, it would be even better if it was a submodule like the other dependencies.

Other than that, heads-up that I'll be on vacation for the coming 3 weeks and will get back to the PR afterwards.

[Tom94]

I'm afraid testing within GitHub workflows won't work -- there's no GPU