Show only one plot label per flowpipe/solution

[schillic]

The consolidation of plot recipes in #933 lead to interpreting a label intended for a flowpipe/solution to be used for each individual reach set instead (example). The reason for this behavior is the way the plot recipe is defined in LazySets. I think it is actually a mistake that we define a recipe for a vector of sets there. Recipes work automatically for vectors. Anyway, the solution proposed here is to use a flag from that vector recipe (which was probably intended for exactly that purpose).