Add a check that there are no / in the units string

[efisher008]

(from CF Github: https://github.com/orgs/cf-convention/discussions/391) which is relevant to the "harmonising canonical units" discussion in cf-convention/vocabularies#238.

[efisher008]

This is to do with how they are entered into the editor. We need them to have inversion -1 notation but some authors use division / notation for their units and it would be useful to correct this automatically.