Incompatible targetV management

[annetill]

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

A feature.

• What is the current behavior?

An exception is thrown in case of incompatible voltage targets of generators.

• 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 voltage target of the first one is kept, and we log an error.

• 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)