Use chebi-utils library

[sfluegel05]

Use the new https://github.com/ChEB-AI/python-chebi-utils library. This should not change the functionality, but moves some ChEBI related tasks to a new repository for easier maintenance and use by other libraries. These features include

• downloading chebi.obo and chebi.sdf files
• processing chebi.obo into a networkx graph object
• processing chebi.sdf into rdkit Mol objects
• matching both to create a labeled dataset with min. X molecules per label
• build a stratified split

[sfluegel05]

I have now adapted ChEBIOverX,removed some unit tests covering functionality in chebi-utils (there are corresponding unit tests in chebi-utils) and adapted other unit tests