Create final makecmap_options to pass to makecmap from existing `makecmap_options` and a colormap function. Used in the vis functions, like vis.subject.morph.native, see the note.
makecmakeopts.merge(makecmap_options, colormap, default_colormap = squash::jet)list of `makecmap_options` or `NULL`
a colormap function or `NULL`
the colormap function to use in case none is found in the other parameters
valid `makecmap_options`