Provide a minimal CI

Summary

In order to get high quality code base, the CI is a precious tool. We should have a CI.

What is the current behavior?

No CI currently exists.

What is the expected correct behavior?

As a starting point, a minimal CI should provide the following features:

• Check the style formatting (see #22 (closed)),
• Build the DAQ client code and RPC modules,
• Launch the automatic tests,
• Build the documentation (and, optionally, push it to the website).

Possible fixes

Create a custom Docker image (in the to be created gemonline-ci-image repository) and use it a the base image for this repository CI, using GitLab CI.