Add uniqueness rule for .name fields

[duncandewhurst]

From Open-Telecoms-Data/cove-ofds#6 (comment):

.phase.name, .status.name, etc, anything .name - could 2 items have different id's but very similar names or even exactly the same names? There isn't a rule for unique names.

I don't think we can enforce unique names using JSON schema, but we could add a rule as part of the field descriptions. I think the rule should be a should rather than a must. Making it a must would mean the name was effectively a unique identifier, and we don't know what edge cases might exist for non-unique names.

Edit: If we add this rule, we can add an additional check to https://github.com/Open-Telecoms-Data/lib-cove-ofds