Improve the computation of invisible nodes used in NetworkGraphBuilder

[So-Fras]

• Do you want to request a feature or report a bug?
  Improvement on current code without changing its behavior.

• What is the current behavior?
  The function getNextDepthVoltageLevels in VoltageLevelFilter is used to find the invisible neighboring nodes of the graph visible nodes. This function is not optimal, especially if a very large list of voltageLevels is passed as an input parameter.

• What is the expected behavior?
  The behavior of the NetworkGraphBuilder should not change but the invisible nodes should be collected in a less resource-guzzling way, by reusing a slightly modified traverseVoltageLevels function.

• What is the motivation / use case for changing the behavior?
  Improve performance / clean code

• Please tell us about your environment:

  • PowSyBl Version: 5.1.1