Added constraints on CodeLabel entity:
- 2 Code labels cannot have the same codeList and key (they are
identical);
- None of the fields "codeList", "key" and "" are nullable.
Implemented equals() and hashCode() in compliance with these
constraints.

Note: The NACE code labels persisted in AwacDummyDataCreator are not
compliant with these constraints (same key for several code labels with
same codeList)...