Development of automated tests

[jnettels]

Currently there are only tests for building the project and testing code style. It would be useful to test the functionality of dhnx.

Instead of writing unit tests for various functions, I would propose to start implementation of tests by automatically running some of the example scripts, like e.g.:
https://github.com/oemof/DHNx/blob/dev/examples/optimisation/import_osm_invest/import_osm_invest.py

We could test whether the code finishes successfully (at all), or e.g. compare an expected length of the network. However, this might change if the downloaded openstreetmap data changes for any reason.

Any thoughts?