Revisit the CI

[aterga]

The current version of Github CI for ViperServer is prototypical. This issue can serve as an umbrella for collecting to-dos in the context of CI.

In particular, we determine a reasonable policy for producing and storing nightly and stable releases. Link to the discussion.

[ArquintL]

One CI improvement is:

I think publishing should only happen if this step is run as part of a cron job (and on master) instead for every single push and pull request. In addition, I would split the job into multiple jobs and communicate via uploading/downloading artifacts

[ArquintL]

Several improvements to CI have been performed with the latest PRs. Thus, I close this issue