[IIDM v1.0] Add getNetwork() to Identifiable

[mathbagu]

• Do you want to request a feature or report a bug?
  Feature
  Implementation of Identifiable::getNetwork() powsybl-core#947

• What is the current behavior?
  There is no way to convenient get the network from an identifiable.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?
  To have a getNetwork() method for each identifiable.

• What is the motivation / use case for changing the behavior?
  Have both Java and C++ implementation as close as possible

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• 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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