[IIDM 1.2] Identifiable.name is optional + introduce getOptionalName(), getNameOrId() + deprecate getName()

[miovd]

• Do you want to request a feature or report a bug?
  Feature

• What is the current behavior?
  Identifiable.getName() never returns null: either it returned the defined name or the id.

• What is the expected behavior?
  name attribute should not be systematic: the API should be clearer about returning the real name or the id. New methods getOptionalName() and getNameOrId() are hence introduced.

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

cf. powsybl/powsybl-core#1243

[sebalaig]

Duplication of issue #124, fixed by PR #142. => closed