Add support for useful iterator memberse and traits in ChannelList::Iterator

[milasudril]

I think the output from gcc summarizes this feature request:

lib/pixel_store/image_io.cpp:42:40: error: no match for ‘operator*’ (operand type is ‘Imf_3_1::ChannelList::ConstIterator’)
   42 |                 for(auto const& item : channels)

Please complete the iterator class with all missing member functions. Since it looks like you use an std::map, you should implement everything needed to satisfy the bidirectional iterator concept.

[Arcmantis18]

I'll look into adding this.

[cary-ilm]

Closing this as resolved by #2384