Add update methods in CgmesModel

[miovd]

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

• What is the current behavior?
  CgmesModel can only be updated or extended by reading CGMES files (in DataSource)

• What is the expected behavior?
  It should be possible to extend or update CgmesModel by adding directly PropertyBags for example.

• What is the motivation / use case for changing the behavior?
  It allows CGMES pre-processing to write files optionally

• Please tell us about your environment:

  • PowSyBl Version: 5.3.0-SNAPSHOT

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

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