ITK conversion

[dave3d]

We should probably have functions to convert images back and forth to ITK-Python.

There is such code in FREDtools here:
https://github.com/jasqs/FREDtools/blob/cd37e14c6bb37d84fe536cd37f229b77415431a2/fredtools/__init__.py#L224

That code uses numpy as a conduit (like our VTK functions).