Provide basic support for older pymatgen versions in adapters

[ml-evs]

See pedrobcst/Xerus#46

If someone has installed pymatgen + optimade-python-tools through means other than optimade[pymatgen], they can end up with silently conflicting versions of the two.

The only place this crops up is in the adapter code. We could add a pymatgen version guard to this code, and potentially even support the old Structure invocation (i.e., not passing dimension_types to the constructor) quite easily, even if we probably cannot test it ourselves. The only other issue is that these old pymatgen versions probably have other conflicting requirements with our dependencies, so if we added this "fix" to the next release, it might still be impossible for people to install the two together (will check this).

[ml-evs]

Closing this for now as the issue resolved itself in Xerus.