Add a convenient function to get a Terminal from its node

[mathbagu]

• Do you want to request a feature or report a bug?
  Feature
  Add UndirectedGraph.vertexExists(node) method + NodeBreakerView.getOptionalTerminal(node) powsybl-core#1235

• What is the current behavior?
  The getTerminal(int node) method of the NodeBreakerView throw an exception if the node doesn't exist.

• 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?
  Another method should be added to get an optional of Terminal if the node doesn't exists.

• 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)

[sebalaig]

Fixed in PR #141