Size and grid fitting cells

[flo-dup]

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

• What is the current behavior?
  Grid is displayed on each side of bus bars even if no cells up or bottom
  SVG size has similar problems

• What is the expected behavior?
  Grid not displayed above (resp. under) the busbars if no cells above (resp. under)
  SVG size fitting for those 2 cases

• What is the motivation / use case for changing the behavior?
  Size / grid fitting to have a nice looking diagram

• Other information
  When drawing the grid, VoltageLevelGraph::getExternCellHeight(direction) can be used.
  For the size I think we only need to adapt PositionVoltageLevelLayout::calculateExternCellHeight to keep 0 value if no BusCell of specific direction