[IIDM v1.1] Add convenient functions to get a network (name or content) depending on the IIDM format version

[mathbagu]

• Do you want to request a feature or report a bug?
  Feature
  add AbstractXmlConverterTest.getVersionedNetworkAsStream method in test powsybl-core#1178
  Add AbstractXmlConverterTest.getVersionedNetworkPath method powsybl-core#1182

• What is the current behavior?
  There is no standard way to get a network's name or content from its short name and a version.

• What is the expected behavior?
  The test API have access to two functions to get the network's name or content.

• What is the motivation / use case for changing the behavior?
  Have both Java and C++ API 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)

(if a question doesn't apply, you can delete it)

[mathbagu]

Already fixed in #80