Add iterator for iterating over channels
Add iterator for iterating over channels

Rather than adding the ability to get the array's by index, provide
a more pythonic interface by using an iterator.

Signed-off-by: Chris Harris <a361e89d1eba6c570561222d75facbbf7aaeeafe@kitware.com>

OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz