[BUG] PYME.IO.rgb_image.image_to_cmy gives all zeros for greyscale

[barentine]

Describe the bug
pretty easy to see in the math why. For single channel it's just:
255 - (255 - data + data)

To Reproduce
Steps to reproduce the behavior:

1. rgb a single channel image, note that it works
2. cmy that same image and note that it gives you all zeros

Expected behavior
check if something is single channel and leave it be in cmy conversion if so

[David-Baddeley]

This looks like it should be fixed