Add color schemes to matpopmod

I do not like the default color palette "tab10", and I think we might do better, at least for stack plots.

The problem of tab10 is that it aims at making colors as easy to tell apart as possible, pairwise. But for stack plots we do not really need this; instead we need adjacent colors to be easy to tell apart.

So I though about:

1. Allowing the user to specify the colors they want (obviously)
2. Providing alternatives to tab10 -- and maybe make one of these alternatives the default!

There are some color palettes from https://colorbrewer2.org that I like, so I though we could implement a few of them (the license allow it and we can cite them).

I did a very quick and botched up implementation of a single color-scheme (the diverging RdYlBl one), and here are a few comments:

It is not always easy to tell the data apart, but when it isn't then honestly it isn't either with tab10. For instance:

RdYlBl:

tab10:

I think that here there is no doubt that both figures are illigible, and both look bad. But I think that tab10 looks more than bad: it looks terrible.

Pretty much the same thing happens with D. sylvestris:

In fact, it's always more or less the same thing: either both figure are easy to read or both are hard to read. And usually I find the RdYlBl one a bit nicer to look at (mostly because of the pastel colors, which are better for large surfaces.

One major argument against implementing the color schemes of colorbrewer is that this is a bit of work. This is because the color palette depends on the number of colors needed (unlike with tab10; but I think that, if anything, it's an improvement over tab10)