`vehicle_logging_timestep_interval=-1` silently zeroes `average_speed`, Edie states, and MFD

[toruseo]

Summary

When vehicle trajectory logging is disabled with World(vehicle_logging_timestep_interval=-1) (the documented memory-saving option), several analyzer outputs silently degrade to zero instead of being reported as unavailable:

• print_simple_stats() reports average speed: 0.0 m/s (it is an incremental mean over vehicle log records, which never accumulate),
• Analyzer.compute_edie_state() produces all-zero k_mat / q_mat,
• consequently macroscopic_fundamental_diagram() draws an empty plot.

Other statistics in the same output block (travel times, completed trips, total distance) remain correct, which makes the zero average speed look like a valid result rather than a missing one.
This affects both the Python and C++ backends equally.

Minimal reproduction

from uxsim import World

def run(cpp, vli):
    W = World(name='t', deltan=5, tmax=3600, print_mode=0, random_seed=0,
              cpp=cpp, vehicle_logging_timestep_interval=vli)
    W.addNode('A', 0, 0); W.addNode('B', 2000, 0); W.addNode('C', 4000, 0)
    W.addLink('AB', 'A', 'B', length=2000, free_flow_speed=20, number_of_lanes=2)
    W.addLink('BC', 'B', 'C', length=2000, free_flow_speed=20, number_of_lanes=2)
    W.adddemand('A', 'C', 0, 3000, flow=0.8)
    W.exec_simulation()
    print(f'cpp={cpp} logging={vli}: average_speed={W.analyzer.average_speed:.1f}')

for cpp in [False, True]:
    for vli in [1, -1]:
        run(cpp, vli)

Output (uxsim 1.14.0):

cpp=False logging=1:  average_speed=20.0
cpp=False logging=-1: average_speed=0.0
cpp=True  logging=1:  average_speed=20.0
cpp=True  logging=-1: average_speed=0.0

(The true average speed is 20 m/s in all four runs; the trips themselves are simulated correctly.)

Suggested fixes (any of these would help)

• Report nan (or omit the line) for statistics that need vehicle logs when logging is disabled, and/or emit a warning from print_simple_stats() / compute_edie_state();
• compute the printed average speed from total distance traveled / total travel time, which is already available without vehicle logs;
• document in vehicle_logging_timestep_interval which analyzer outputs become unavailable when it is set to -1.

(found and reported by my agent)