refactor subset expressions in plot3d.neuronlist

* this now relies primarily on subset.neuronlist
factor out colour handling code of plot3d.neuronlist
plot.neuronlist method

* but only works for neurons at the moment
Normalise arguments for plot(3d).neuron
Teach plot.neuron not to plot any points at all

* i.e. just the lines
Merge branch 'feature/plot.neuronlist'

* feature/plot.neuronlist:
  Teach plot.neuron not to plot any points at all
  Normalise arguments for plot(3d).neuron
  plot.neuronlist method
  factor out colour handling code of plot3d.neuronlist
  refactor subset expressions in plot3d.neuronlist

* closes #113 and #102 though some more work may be necessary in addition to
  this basic implementation.