warn for unknown element symbols

[ltalirz]

Another point discovered with CIFs from the CSD: pymatgen spits out the symbol "D0+" when the structure contains deuterium, which is then not understood by zeo++

Unable to find radius for D0+ in table. Please provide it

zeo++ actually has a radius for deuterium, so it's not really its fault (more pymatgen's).

However, it might make sense to add a check in aiida-zeopp in prepare_for_submission:
if no radiifile is provided, one could try to parse the CIF file and check whether there are any element symbols unknown to zeo++ (see link above for the known ones).

In any case, this issue is mostly for reference - it's probably best to fix this at the pymatgen side