Investigate into PyPSAExample Coverage Problem in the TestPowerGrid Code

[franziska-wegner]

The power grid data structure tests have problems in the coverage workflow with the PyPSAExample data example. For now, we excluded these tests. Note that the tests itself do not fail, only the accumulation process of the gcov execution. Further investigation is required. It might be due to the file destination that differs from the Matlab examples. See PR #53 for more information.