initial setup gitlab-ci pipeline

This MR sets up the gitlab-ci by adding the file .gitlab-ci.yml. Currently, only one stage is included in the ci pipeline: tests. This stage has to jobs

Unit tests

This job is triggered if an MR targets the long-lived branches dev or main. It runs just the unit tests defined in minterpy/tests resulting in a report.xml. This shows the test results directly in the MR.

Coverage tests

This job is only triggered if a commit is done to main. It disables the numba support and runs the unit tests with the coverage flag. This enables the coverage to be shown as a badge in the readme later (this will follow in another MR).

Final remarks

This MR is the starting point of a series of MRs to set up the ci. The remaining open tasks for the CI will be tracked in #60.