Make SLD and NAD APIs more consistent

[So-Fras]

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

• What is the current behavior?
  SLD and NAD APIs are different.
  For example, in order to draw a diagram with SLD, the following code is used:

SingleLineDiagram.draw(...);

When generating a diagram with NAD, one has to do the following:

NetworkAreaDiagram nad = new NetworkAreaDiagram(...);
nad.draw(...);

• What is the expected behavior?
  Both APIs should work the same for the draw() function and be as consistent as possible for the rest of the code.

• What is the motivation / use case for changing the behavior?
  This will help users to master both APIs more easily.

• Please tell us about your environment:

  • PowSyBl Version: 3.0.0