Stacking snake lines

[flo-dup]

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

• What is the current behavior?
  There's always layoutParam.getVerticalSnakeLinePadding() between two consecutive snake lines between voltage levels, even if no crossing would occur.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
  See the example of the tutorial, the two first snake lines above could be at the same ordinate, leading to a more compact diagram.

• What is the expected behavior?
  Two snake lines which do not overlap could be at the same ordinate,

• What is the motivation / use case for changing the behavior?
  More compact diagram

• Other information
  A way to do it could be to know which are the "slots" available, in fact it reminds me of the skyline problem, maybe the similarity could help finding a nice algorithm?

[BenoitJeanson]

There is an algo that does something similar for internCells in BlockPositionner:InternCellsLanes::arrangeLanes
inspiration or reuse?