Build RDataFrame graph after definePlots (and reorder for efficiency)

This is just an experiment, pushing to allow wider testing and collect feedback (if there is interest a bit of cleanup and refactoring will be needed).

Since in this case the backend automatically performs the definition of RDataFrame nodes in the "efficient order" from here, the user code does not need to do so any more (something both @swertz and @fbury indicated could be useful).

To test it should be sufficient to pick up this change and add lazyBackend=True to the call to super(..., self).prepareTree.