Setup CI and unit tests #4
Description
We should add some unit tests to the package. These can be inspired by the tests in MetopDatasets.jl to unsure that MetopPy is consistent with that. The main focus is just to test the interface of MetopPy with MetopDatasets. The core logic of the reader is already tests in MetopDatasets and we don't need to duplicate it all.
The package should also have CI setup to run unit tests on new PRs.