Fix channel selector when duplicate channels present (resolve #11)

[alanocallaghan]

Asking for @petebankhead to have a look as this type of behaviour should(?) also be fixed in the pixel classifier, I'd say?

[petebankhead]

Erm, not sure. I think with the pixel classifier, the intention was to store the channel name in the json - not the number. Since the channels might get shuffled. Or, indeed, names might not be parsed properly. Using the name makes this much more easily correct-able than requiring the image to be rewritten with channels in the 'right' order.

Clearly the use of names doesn't handle duplicates, but if you've got duplicates then Something Bad Has Happened anyway.

[alanocallaghan]

Yeah, maybe the solution is a more strongly-worded warning for anything that tries to use channel names when there are duplicates. Though I guess we'd want to be able to point to a solution (python/groovy script to rename channels, maybe?)

[alanocallaghan]

Added some restructuring, but generally this should be a good setup for scripting and interactive use