Add support for setting palettes and colour transfer functions

[zmughal]

For example, for plotting images.

[zmughal]

Also useful for complex numbers: https://en.wikipedia.org/wiki/Domain_coloring.

[mohawk2]

Oh, good point! I'd forgotten about this issue :-D

[mohawk2]

Having pondered some more, I think P:G:S should have a way of setting the palette in a fairly dumb way, and that the domain colouring itself would live elsewhere, possibly in main PDL, or in PDL::Transform::Color, with outputs that can be consumed by the palette-setters.