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
sty: single line for imports
provide examples
db arg now has default value of NULL

* this is more convenient when calling from other functions
add explicit db argument
clarify extrafun args
teach nlscan to work directly with neuronlists

* adjust examples accordingly
Run one nlscan example
fix bug in nlscan when wait=F
Merge branch 'feature/nlscan'

* finalises #167
* feature/nlscan:
  fix bug in nlscan when wait=F
  Run one nlscan example
  teach nlscan to work directly with neuronlists
  clarify extrafun args
  add explicit db argument
  db arg now has default value of NULL
  provide examples
  sty: single line for imports
doc: correct return value of nlscan
docs: note about base set functions
Prepare version 1.5.14