Make unit tests self-contained

[CasperWA]

Since the dic2owl tool downloads various dictionaries it needs for the ontology generation the testing is not self-contained.

To avoid relying on external online resources for testing, either the needed files should be added to the tests/dic2owl/static folder and provided when Python tries to download the various files - or the COMCIFS/cif_core repository should be added as a submodule, since this is where the files are from.

For the latter solution we would potentially always be testing against the latest files (as long as the submodule is updated). Furthermore, the files would not become an actual part of the repository as long as the submodule is not initiated, which it would only need to be for testing purposes.