docs: typo
fix: writing neurons in subdirs when df is null

* when neuronlist did not have an attached data.frame but you set subdir=
  expression to be evaluated in the current environment (rather than in the
  context of the attached data frame) then there was an error
* +test
* closes #133
give xform.neuronlist a subset argument

* so that it only xforms a subset of the neurons 
  in the original list.
* sample use case would be mirroring some members 
  of a neuronlist onto a canonical side of the brain.
* closes #134
give mirror.neuronlist a subset argument as well

* and make sure it is used
Add function to plot bounding box in 3D
Merge pull request #136 from ajdm/feature/plotboundingbox

Add function to plot bounding box in 3D
sty: tidying neuron-io test file
Check for file existence in read.im3d before reading

* throwing an error on failure
* this has the side-benefit of suppressing a warning in test for reading 
  non-existent file
Don't write invalid radius information

* warns when there are NAs in radius
* Amira fails to read files with NAs/NaNs
* +test
Merge branch 'master' of github.com:jefferis/nat

* 'master' of github.com:jefferis/nat:
  Add function to plot bounding box in 3D
roxygenise change 3e1dc5ed to export plot3d.boundingbox
Fix xformpoints.cmtkreg when input contains NAs

* fixes #132
* includes test
Install R, X11
Remove recommendation to use lab R repository

* master is pretty stable so just having CRAN and github should be fine
* also suggest using the github release branch
Trigger rebuild of nat.templatebrains on successful Travis build
Give plot3d.neuronlist a SUBSTIUTE argument

* this allows other functions to control non-standard evaluation

see http://adv-r.had.co.nz/Computing-on-the-language.html#calling-from-another-function

for discussion
Test controlling non-standard evaluation in plot3d.neuronlist

* as other functions might do
Give plot.neuronlist a SUBSTITUTE argument as well
remove non-existent parent.generations argument

* closes #140
Pass dots extra args to filterfun

* closes #139