CI Improvements

Changes to be commited:

• Changes to API installation with CMake.
• New Dockerfile
• Nix files for development shell (still need to be improved)
• New CI stages (refactoring .gitlab.yaml)

To be done later:

• Separate CI for SLURM clustering with Docker: https://github.com/giovtorres/slurm-docker-cluster
• Possibly extend similar approach to OAR (Not as important as it runs OpenMPI after submissions anyway.)