Multiple colour schemes in multiplot

[TomReichardt]

This is my first pull request, please be kind.

[danieljprice]

I had a look at merging this. General idea and code looks good, I found a few issues in practice. Mainly that with tiled plots the colour scheme can change but there is a shared colour bar which only shows one scheme. Also, I think it makes more sense to have one colour bar per quantity rather than per plotting panel, which changes the logic in how the array is used. So I have not merged it into the 2.9.1 release, but with a bit of effort I will try to get it in the next one.

[TomReichardt]

Mm, sure. That issue makes sense. I'd have to look into a way to spit up a colourbar if the schemes were not the same.
I did think about doing it per quantity, but I felt that per panel was a bit more general. You can effectively do a colourbar per quantity this way, but if you limit each quantity to strictly having their own colour scheme, then you can't render the same quantity in two different ways. Not sure why you'd WANT to do that, in practice, but somebody might. I guess one could use your workaround of creating a new quantity identical to the old one...