GLMakie: Annotate cell/boundary region color bar

[pjaap]

After using gridplot with GLMakie for a while I asked myself why

• the colorbars display a wrong number of regions (see Use concrete number of grid regions GridVisualizeTools.jl#9)
• the bars are not labeled with a title.

I think the users may use the grid plot to determine the indices and total number of cell/boundary regions by looking at the gridplot.

With this proposed changes, together with WIAS-PDELib/GridVisualizeTools.jl#9, the result looks like this

[pjaap]

2D example:

[j-fu]

I left out these text labels for reasons of screen real estate for the case we have several plots in one window (via the layout stuff). May be we could make these optional, or even make the colorbars optional (which I guess is easier to organize).

Also: how do PyPlot and PlutoVista behave ? We can safely ignore Plots, because this misses decent handling of triangles - at least when I looked last time.

[pjaap]

I understand that the labels require some space. However, without a label these bars are not suitable for scientific plots. I would suggest to include the labels and add a kwarg to deactivate the display of the color bars.

[pjaap]

We should discuss the issue with the concrete number of labels at WIAS-PDELib/GridVisualizeTools.jl#9.

[pjaap]

Rebased. This is now ready.

[pjaap]

Rebased again on top of #39.