plt.pause(0) waits for ever: PoseGraph.plot_scan(pause=0) and Vehicle animation may hang #876

plt.pause(0) does not return: Matplotlib treats a timeout of zero as "wait until the event loop is stopped", so it blocks for ever, with no message. PyPlot.step(0) and PyPlot2.step(0) did this and now raise ValueError (#875). Other places pass a caller's value straight to plt.pause:

• PoseGraph.plot_scan(n, pause=0) (mobile/PoseGraph.py): plt.pause(pause) with no guard, pause is an argument and defaults to 1. Needs a guard, or a ValueError, as in fix(plot): PyPlot and PyPlot2 step() raise ValueError for a dt of zero or less #875.
• Vehicle animation (mobile/Vehicle.py, _refresh_animation_frame(fig, dt, pause)): ends with plt.pause(dt); check whether dt can be zero from the callers (lines 853 and 1841) and guard it if so.

Not affected: PlannerBase.animation_frame already tests pause > 0, and the others pass a constant.

I believe the first two hang the same way but have not run them. A test should use a timeout so that a regression fails instead of hanging (see tests/test_step_dt.py in #875).