Improve CI pipeline

Improve the CI pipeline. Fix the issue that the deploy job would be run before all the tests had run.

To accomplish this, the following changes were made:

• The download_data job was moved to a new stage prepare_tests
• For clarity, the stage tests was renamed to run_tests
• Removed all uses of the needs keyword. Rationale: needs "screws up" the order in which jobs are run, basically ignoring the different stages; note that this is by design.