Display multichannel image

In microscopy, often images have multiple color channels, each having the fluorescence intensity of a certain dye emission. The function `to_rgb` converts these multichannel images to RGB colors. Either the metadata `color` field is used for the colors, or default colors cyan / green / magenta / red.

This is implemented in the `_repr_html_` function, to allow direct views of multichannel images.

Earlier on, I used http://www.noah.org/wiki/Wavelength_to_RGB_in_Python to convert emission wavelengths to RGB values. Because licensing is unclear and I get no response on my emails, I removed this code and changed the wavelength designation to matplotlib colors such as `'r'` or `[0.2,0.0,1.0]`.
I took the ones with the highest luminosity (as calculated with 0.2126 R + 0.7152 G + 0.0722 B), which still have reasonable separation in perceived color. Issue on e.g. beamers is always the low luminosity, my experience is that green, magenta and cyan work best on black backgrounds.

The order is just the most conventional colors used, Green for one channel, Green/Red for 2 channels, Blue/Green/Red for 3 channels and Blue/Green/Red/Far Red for 4 channels. Red is displayed as magenta and Blue as cyan.

But I am open for suggestions, of course.