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?

No.

What kind of change does this PR introduce?

Refactoring.

What is the current behavior?

There is a lack of consistency in the methods of CgmesModel that fetch data based on a common object ID. The methods don't have a similar naming, nor do they have a similar signature, and nor are they implemented in the same way and in the same class.
For instance:

• ratioTapChangerTable (table points grouped by table) // groupedTransformerEnds (transformer ends grouped by transformer)
• PropertyBags ratioTapChangerTable(String tableId) // Map<String, PropertyBags> groupedTransformerEnds()
• ratioTapChangerListForPowerTransformer (caching is done in Context) // nonlinearShuntCompensatorPoints (caching is done in AbstractCgmesModel)

What is the new behavior (if this is a feature change)?

Interface CgmesModel only provides full queries.
Result from full queries is grouped by common object id and cached in Context.

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)

Methods removed from CgmesModel:

• Map<String, PropertyBags> groupedTransformerEnds()
• List<String> ratioTapChangerListForPowerTransformer(String powerTransformerId)
• PropertyBags ratioTapChangerTable(String tableId)
• List<String> phaseTapChangerListForPowerTransformer(String powerTransformerId)
• PropertyBags phaseTapChangerTable(String tableId)
• PropertyBags nonlinearShuntCompensatorPoints(String shuntId)

Have been replaced by these methodes in Context:

• public PropertyBags transformerEnds(String transformerId)
• public PropertyBags ratioTapChangers(String transformerId)
• public PropertyBags ratioTapChangerTablePoints(String tableId)
• public PropertyBags phaseTapChangers(String transformerId)
• public PropertyBags phaseTapChangerTablePoints(String tableId)
• public PropertyBags nonlinearShuntCompensatorPoints(String shuntId)

Also, in CgmesModel, the following methods have been renamed for more consistency:

• ratioTapChangerTablesPoints() into ratioTapChangerTablePoints()
• phaseTapChangerTablesPoints() into phaseTapChangerTablePoints()

Other information:

Early developments of separation of CGMES import (EQ separately from SSH) show the need for caching result of queries in the Context. This refactoring goes in the same direction.