Use pytest framework

In order to obtain a clear suite of (integration) tests, this MR changes to pytest as a testing framework (rather than nothing/unittest).

NOTE:

• the only drawback of changing to pytest is that the preparetestset function (https://git.astron.nl/RD/idg/-/blob/use-pytest-framework/test/integration/gridding/test_gridding.py#L155) is ran both by the test_gridding as well as the test_degridding test. This is slightly inefficient
• the python code for the integration tests is not super-polished. We can pick this up as a minor task later on.

@tol : just curious whether you think/agree that pytest offers more "clarity" compared to unittest