Failing test for plot3d.hclust(, col)

* problem is that plot3d.neuronlist cannot find kgroups from the top of
  the call tree
* this problem comes about because nat::plot3d.neuronlist wants to
  evaluate the `col` and `subset` arguments looking up symbols both in
  the calling environment and the data frame attached to the neuron list.
  parent.frame() is used to see the calling environment, but it may not
  be the
Fix bug in col in plot3d.hclust

* nb depends on nat >= 1.5.6 which has a change in plot3d.neuronlist
Add full example for plot3d.hclust