Add function to screen neurons in 3D
rename screen_neurons to nlscan
remove unused flip argument

* was only relevant with plot3dfc
Change to use substitute() and SUBSTITUTE=FALSE
Merge branch 'feature/screen_neurons' of github.com:ajdm/nat into feature/nlscan

# By James Manton
# Via James Manton
* 'feature/screen_neurons' of github.com:ajdm/nat:
  Change to use substitute() and SUBSTITUTE=FALSE

Conflicts:
	R/neuronlist.R (due to removal of flip arg)
Merge branch 'feature/nlscan'

* feature/nlscan:
  Change to use substitute() and SUBSTITUTE=FALSE
  remove unused flip argument
  rename screen_neurons to nlscan
  Add function to screen neurons in 3D