Review colorbar creation with viz and viewer

[juliohm]

We need to review our treatment of colors in viz and viewer. In particular, we need to look into two areas:

1. Makie now supports a Makie.Categorical(colorscheme) to convert a continuous colorscheme into a categorical colorscheme with good tick defaults. We need to use that instead of placing our ticks manually.
2. When using fig, ax, plt = viz(...) it is not always possible to create a colorbar automatically from the plt object, because of missing attributes such as colormap, colorrange, etc. This automatic creation of colorbar is only possible in some Makie builtin functions because they store these attributes.

[juliohm]

Fixed on master.