#2647 - Revise plot recipe for UnionSet

[schillic]

Closes #2647.

• The UnionSet and UnionSetArray recipe now just plots a sequence of sets with the same options (in particular the same color and a single label).
• There was a plotting argument fast but using it was not comfortable, so I turned it into a keyword argument.
• I also changed the name of the fast argument to same_recipe because that describes the effect better. That it is faster is a nice side effect but less important.

[schillic]

Plotting flat sets with this same_recipe option results in black line segments. This is because we do not set the line color by default and this plot recipe does not call the special "flat set" recipe. I suggest that we set the line color to the same color.

EDIT: We agreed that the border color should remain black by default. If needed one can use plot(...; same_recipe=true, linecolor=...).