Re-enable optional rocrate dependency in released version

Summary

Currently our rocrate implementation depends on @salexan's fork which would violate PEP440 (see https://github.com/pypi/warehouse/issues/7136), so we can't upload the crawler package to PyPI without removing the optional rocrate dependency first.

Once the upstream issue, which made the fork necessary in the first place, has been resolved, we can re-enable it.

Until then, users who want to use the ROCrate or ELN File converters can either install the crawler in the dev branch from this repository (which has the dependency enabled) or manually install

pip install git+https://github.com/salexan2001/ro-crate-py.git@f-automatic-dummy-ids