doc: more details for plot3d.neuron(,soma)
Give plot3d.neuronlist a soma argument

* this is used directly for dotprops objects and passed on to plot3d.neuron
* TODO would still be more elegant if dotprops objects actually contained the
  soma information. This would allow e.g. mixed neuronlists
Merge branch 'fix/plot3d-soma'

* fix/plot3d-soma:
  Give plot3d.neuronlist a soma argument
  doc: more details for plot3d.neuron(,soma)
* fixes #56
Make .plotted3d environment on startup

* will be used to store info about last plotted neurons
Add npop3d and teach to plot3d.neuronlist to prepare for it

* identical to flycircuit::pop3dfc, except that it works with any
  neurons plotted by plot3d.neuronlist, rather than only dotprops
  objects plotted by plot3dfc
Add test for npop3d
Merge branch 'feature/npop3d'

* feature/npop3d:
  Add test for npop3d
  Add npop3d and teach to plot3d.neuronlist to prepare for it
  Make .plotted3d environment on startup
* part of fix for jefferis/flycircuit#11
Bump version to 1.5.7