Add support for RGB images

[PierreRaybaut]

DataLab provides image processing features for many data types but only grayscale. Currently, here are the supported data types: uint8, uint16, int16, float32 and float64.

This issue aims at proposing a new RGB data type which would be for example based on uint8.

This follows #118 reporting issues with image color spaces while trying to import a RGB image into DataLab.