Automatic extension of colorbar

[natgeo-wong]

Hi, as per advice a few days ago, I would like to open this issue for colorbar extensions to automatically extend the colormap for discrete colormaps.

Currently, the :auto: option for extending the colorbar results in the following:

But I would like something like this instead:

Where the colormap is automatically extended to fit.

[briochemc]

Just to be clear for the record here, the current issue is that in the discrete case, the :auto option uses the same color for the extension and for the last level inside the color range.

For instance, the issue in the 1st image above is that the color between +4 and +5 is the same as the color above +5.

Instead it would be better to stretch the colormap (by one or two entries, depending on whether one or both extensions are requested) and then ensure that the extensions are colored with the next/previous color, as in the second image.

[jkrumbiegel]

Should be fixed by #2183

[jkrumbiegel]

An example from the docs