Changes after line notes

- added clipping step, when colors overlap too much, renormalizing would
reduce the luminosity of the channels. When clipping needs to be
avoided, the user can specify his own RGB values (see docstring)
- when normalize == True, also monochannel images are normalized
- moved import ColorConverter to module level
- defaults colors are now directly rgb to circumvent ColorConverter
- more efficient _monochannel_to_rgb