Add a volume rendering widget and example.
Change example so that it doesn't require input file.
Add docstring
Merge pull request #5 from enthought/feature/simple-volume-rendering-example

Simplify volume rendering example
Don't build method names for FileDialogEx
Reduce code duplication in the volume mapper setup.
VolumeData spacing should default to (1,1,1)
Changed the default dimensions of the resampled volume to 256^3
Remove unnecessary dictionaries.
Merge branch 'master' into feature/volume-rendering
Add a very basic unit test for the volume renderer widget.
Merge pull request #2 from enthought/feature/volume-rendering

Add a volume rendering widget and example