New Continuous Integration

[LourensVeen]

Is your feature request related to a problem? Please describe.
The old CI wasn't working too well, and didn't cover all codes. It's been updated a bit to work with the new build system, but it's still hanging on Linux and it has MPI issues from time to time. We really need a reliable CI to pull us out of the current broken windows situation.

Describe the solution you'd like
A new CI setup with:

• Checks on every PR that run the framework tests and tests for any codes that have changed
• Regular builds on a variety of systems which build everything and run the tests
• Something with a GPU so that we can run tests for the GPU codes as well

Describe alternatives you've considered
We're currently having MPI issues with the free GitHub Actions runners, and they don't have GPU support. We could consider going with paid GitHub runners, an external service (Ubicloud seems like a good option), or a dedicated physical machine in Leiden with a GPU in it. A service would have the advantage that you don't need to maintain it and it's more energy efficient, a local machine would be more flexible and allow easier local experimentation. Cost would have to be considered too.

[LourensVeen]

Update on this: I've managed to make the GitHub CI a bit more reliable (see #1260), and we have a local machine at the Sterrewacht with 4 cores, 16GB RAM and a GPU.

I'll finish the workflows first, then try to connect that machine to GitHub (and BitBucket, for Torch, but that's another story).