Add support of color image @interpretation

[t20100]

If we want to be fully compatible with nexus, we'll have to handle more @interpretation for color images: See https://github.com/nexusformat/definitions/pull/834:

			* ``rgb-image`` (3-D data)
			* ``rgba-image`` (3-D data)
			* ``hsl-image`` (3-D data)
			* ``hsla-image`` (3-D data)
			* ``cmyk-image`` (3-D data)

Question: Do we want to support that?

[vasole]

Well, rgb and rgba are already supported.

For the others I see little interest.

[t20100]

Strictly speaking only @interpretation="rgba-image" is supported even if it already supports both RGB and RGBA images.

We do not understand: @interpretation="rgb-image" and as I commented in nexusformat/definitions#834, to me it is not needed, the shape of the data already provides this information.