[IIDM v1.2] Improve performance

[mathbagu]

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

• What is the current behavior?
  Since the merge of [IIDM V1.2] Removed duplicates in VoltageLevel::getConnectables (#46) #143 we notice a performance issue ([IIDM V1.2] Removed duplicates in VoltageLevel::getConnectables (#46) #143). The export is 30% slower due to the use of DistinctPredicate. To export a VoltageLevel, the list of terminals is visited multiple times that is not efficient.

• 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?
  Better performance

• 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)

[mathbagu]

Fix in #225