CGMES: remove extension for Control Areas, use IIDM Area

[zamarrenolm]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

What kind of change does this PR introduce?

Update

What is the current behavior?

Information from CGMES control areas is stored using an extension to the Network Model (CgmesControlArea).

What is the new behavior (if this is a feature change)?
Information from CGMES control areas is stored using native IIDM objects of class Area.
CGMES pTolerance attribute is stored as a property.
The ENTSO-E attribute IdentifiedObject.energyIdentCodeEic is stored as an alias.

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information: