Network variant manager implementation

[geofjamg]

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

• What is the current behavior?
  Variant management is not yet implemented. The following code throws a TODO exception:

  network.getVariantManager();

• What is the expected behavior?
  We must implement variant management. Ideally variant could also be structural variant unlike original IIDM impl which only support state/control variable variants.

• What is the motivation / use case for changing the behavior?

• 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)