Implement parsing of axes for 4D data (but don't actually load it)
Implement parsing of axes for 4D data (but don't actually load it)

* This still works for 3D data,
* BUT will load a garbled subset of 4d data into a scalar field
* but at least it should get the bounding box correct

jefferis/hxNrrdIO,jefferis/hxNrrdIO