Plotting improvements

[Warggr]

I added a few features that might be relevant for other users:

• plot_x can plot systems of any dimension.
• When the constraints are unbounded in one dimension and do not have vertices (e.g. $\{ x_1, x_2: -1 \le x_1 \le 1 \}$), lines are drawn for the dimensions that are bounded (previously, this resulted in an error)
• plot_constraints is its own function - this allows users to import plot_constraints and plot constraints on their own figures.

This does not break previous code (as far as I can tell; at least the tutorial notebook still works).