finalizing the Iterator API

[Casper Boemann]

> Note that you often need quite different rendering code for the same basic
> colour strategy with different channel sizes -- check the way exrdisplay
> calculates the display pixels for a 16-bit image in OpenEXR 1.2.0.

Templates allow typespecific parts. I know you asked this earlier, but look
for specialization of templates in any comprehensive C++ book. I even have a
way of doing this without using this feature, should the compiler not
support it.