Closes #113 - Add Chebi (Chapti)

[napsternxg]

Please name your PR after the issue it closes. You can use the following line: "Closes #ISSUE-NUMBER" where you replace the ISSUE-NUMBER with the one corresponding to your dataset.

Fixes #113

If the following information is NOT present in the issue, please populate:

• Name: chebi
• Description: http://chebi.cvs.sourceforge.net/viewvc/chebi/chapati/
• Paper: https://europepmc.org/articles/PMC2238832
• Data: http://chebi.cvs.sourceforge.net/viewvc/chebi/chapati/

Checkbox

• Confirm that this PR is linked to the dataset issue.
• Create the dataloader script biodatasets/my_dataset/my_dataset.py (please use only lowercase and underscore for dataset naming).
• Provide values for the _CITATION, _DATASETNAME, _DESCRIPTION, _HOMEPAGE, _LICENSE, _URLs, _SUPPORTED_TASKS, _SOURCE_VERSION, and _BIGBIO_VERSION variables.
• Implement _info(), _split_generators() and _generate_examples() in dataloader script.
• Make sure that the BUILDER_CONFIGS class attribute is a list with at least one BigBioConfig for the source schema and one for a bigbio schema.
• Confirm dataloader script works with datasets.load_dataset function.
• Confirm that your dataloader script passes the test suite run with python -m tests.test_bigbio biodatasets/my_dataset/my_dataset.py.
• If my dataset is local, I have provided an output of the unit-tests in the PR (please copy paste). This is OPTIONAL for public datasets, as we can test these without access to the data files.

[mariosaenger]

@phlobo Please have a look at the implementation.