Added non-continuous gradients from colorbrewer

[ValentinKaisermayer]

At least for line plots this seems better than https://github.com/JuliaPlots/PlotUtils.jl/pull/67

[daschw]

Thanks! I think

for sym in (:Set1, :Set2, ...)
    _themes[sym] = PlotTheme(
        palette = expand_palette(colorant"white", cgrad(sym).colors; lchoices = [57], cchoices = [100]),
    )
end

should also work as a shorter implementation. It uses cgrad from PlotUtils

[daschw]

... but requires a new release from PlotUtils.

[mkborregaard]

I don't think this is the right place to add the palettes - we should fix this in PlotUtils

[daschw]

Right, then one could just do theme(palette = :grad) or default(palette = :grad) for any :grad defined in PlotUtils.

[BeastyBlacksmith]

I think this is also resolved by the ColorSchemes integration