[IIDM v1.0] Fix implementation of TapChangerStep

[mathbagu]

• Do you want to request a feature or report a bug?
  Feature
  iidm notify update on TapChangerStepImpl powsybl-core#913

• What is the current behavior?
  The TapChangerStep doesn't have its position.

• 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?
  The TapChangerStep should know it's position.

• 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)

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

[mathbagu]

The position of the TapChanger is used for notification, but we don't have this feature. Maybe we can postpone this task