Rename topdownpacking.nodePlacement

[soerendomroes]

The option topdownpacking.nodePlacement.strategy collides with the already existing layered.nodePlacement.strategy.
Since nodePlacement.strategy is a commonly used in example models, these break since we need to specify that the layered node placement strategy is meant.

I suggest renaming the topdownpacking.nodePlacement.strategy to something else (which is not placement or noedPlacement since both are already in use by rectpacking and layered.

@Eddykasp