Consider moving channel_convert_to_unsigned out of detail namespace

[mloskot]

From @marco-langer #703 (comment)

Regarding this channel_convert_to_unsigned, which is now in the detail namespace, but also still referenced in this tutorial:

Maybe one should think to pull it out of the detail namespace again, if it is that useful for client code? I don't know the rational why it was moved into the detail namespace at all (this was done 15 years ago!).

I think, it is worth to keep this discussion open and come back to it in future.

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For the records, it moved into detail in 2ad6747
which refers to the "GIL 2.0 to GIL 2.1 Changes" PDF. Since I have got this gil2.1_changes.pdf, I've attached it for the reference. It does not explain the move rationale though.

I attached also earlier gil2.0_changes.pdf which lists lots of changes related to Boost'ification of the Adobe GIL library. This makes me guess that moving of the types into the detail namespace may be related to the unification of GIL according to Boost conventions.

(both legacy documents available at https://github.com/boostorg/gil/wiki/History)