Add support for rank/node distinction with multi block dataset

* When the configuration file specifies `n_ranks` --> change nothing.

* New feature: when **instead**
```
n_nodes = <some int value>
n_ranks_per_node = <some other int value>
```
--> produce a `vtkMultiBlockDataSet` output which has `n_nodes` blocks, each of which is a `vtkPolyData` instance, similar to what we currently have in vt-tv