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