Adding color deconvolution for immunohistochemical images

Immunohistochemical images are most often stained with Haematoxylin (blue), Eosin (pink) and DAB (brown; chromogen used in immuno reactions enabling to visualize the presence of a protein).

The code in this branch enables the separation of the contribution of the aforementioned stains to the intensity of the image. The result is an image with "haematoxylin", "eosin" and "dab" channels. Nucleus and staining detection on this image is therefore much easier...

I added an hed2rgb function, essentially for validation purpose, since this is of limited interest in practice.