Fixes and improvements to palettes and LinearColorMapper

issues: fixes #1545
> What kind of statistics? 

Personal preferences statistics :-)

I think that relying on `docstring` for a class/function definition to be correct is a weak pattern (`pass` instead is a clear *does nothing* statement). IMHO it seems more an implementation [niche] detail then a strong language idiom (but I may be completely wrong about this).

That said, it's just a personal preference and I agree it doesn't deserve more then 2 minutes... :wink: