Hello @bleykauf

In the past we talked about using Systemd instead of screen for managing the server. This PR suggest a way to accomplish that, while still making most regular users happy with the deployment experience. No changes were made to the README.md yet.

1st commit: linien-gui: Allow to avoid imperative installations via env

I added a simple os.getenv check that allows running the GUI in a way that will disallow running linien_install_requirements.sh on the server.

2nd commit: server: Use systemd .service file instead of screen

I removed the linien_{start,stop}_server.sh scripts, in favor of a linien.service file that should be installed on the server in /usr/lib/systemd/system directory, if linien_install_requirements.sh was executed, or if an advanced user has installed that file there themselves (Using Nix for instance, see next commit).

3rd commit: Add nix files & relax rpyc dep versions

I wrote nix files (their main file is flake.nix) that defines the Nix packages that can be built using nix build. More importantly, it is now possible to cross compile a linien-server debian package using:

nix build --print-build-logs .\#linien-server-deb-armv7l-hf-multiplatform

BE CAREFUL, it takes a long time and a lot of CPU and memory. But eventually, you should be able to copy that deb file to your redpitaya using:

scp $(readlink $(nix build --print-out-paths --print-build-logs .\#linien-server-deb-armv7l-hf-multiplatform)) redpitaya:.

This debian package has all of the dependencies needed for linien, and it doesn't depend on anything installed on the target redpitaya besides systemd - an example of the power of Nix.

Lastly, I encountered some issues with the rpyc 5.0 dependency. As I mentioned in linien-org/pyrp3#10 . The Nix installation takes care of avoiding that, but ideally this should be removed from the relevant setup.py files.