speed up output generation, especially VTK based output
The VTK output spends a lot of time going through the object tree. It could do this differently, as discussed in #62 See doussan code which holds a graph of the whole root system in memory.
The VTK output spends a lot of time going through the object tree. It could do this differently, as discussed in #62 See doussan code which holds a graph of the whole root system in memory.