Make the OperationalLimitsGroup id globally unique

[rcourtier]

Describe the current behavior

Following #3139, at CGMES import the IIDM OperationalLimitsGroup id is copied from CGMES OperationalLimitSet rdfid. This is neccesary to ensure a strict 1:1 mapping, as well as be able to match limit values update in SSH with limit creation in EQ.

This has however a limitation: the IIDM OperationalLimitsGroup id is by design not globally unique (it is only unique per branch end/transformer leg). Sure it gets unique values during CGMES import since rdfid are globally unique, but since OperationalLimitsGroup can be created directly within IIDM or from other kind of imports, there is no guarantee they are unique. Because of that, it is not possible to directly map IIDM OperationalLimitsGroup id to CGMES OperationalLimitSet rdfid at CGMES export.

Describe the expected behavior

The expectation is that the IIDM OperationalLimitsGroup id is globally unique

Describe the motivation

The change would simplify the CGMES export, making a direct mapping possible between IIDM OperationalLimitsGroup id and CGMES OperationalLimitSet rdfid.

Also, in conjugation with #3152, there would be no need for the CGMES.OperationalLimitSet_<rdfid>/<name> property attached to the identifiables holding limits in IIDM anymore.

Extra Information

See #3152 for adding a name property to IIDM OperationalLimitsGroup.