Invariant not implemented in class TermMapping

[Sergio Miranda Freire]

The invariant
Purpose_valid: purpose /= Void implies terminology(Terminology_id_openehr).
has_code_for_group_id(Group_id_term_mapping_purpose, purpose.defining_code)

is not implemented and it must use the openehr terminology.

We must be establish a policy on how to implement invariants, for instance: declare the invariant in the interface and implement it in the class that implements that interface.