BibTex to olca-schema.Source translation: generate URL from DOI

[a-w-beck]

The olca-schema's Source object has no doi field, but it does offer Source.URL.

Please modify bibtex.py to populate url with a valid URL generated from the doi (e.g., f'https://doi.org/{doi}') wherever a DOI is available, even overwriting already-present URLs, since the new DOI URL should redirect to the same page and uses fewer characters.