Catch wrong parameters for sparse_rail_generator
If only one city can be created we have to abort or catch that error.
Currently this is done in rail_generator.py.
if num_cities < 2:
warnings.warn("[WARNING] Changing to Grid mode to place at least 2 cities.")
city_positions = _generate_evenly_distr_city_positions(max_feasible_cities, city_radius, width,
height)
We should handle this exception differently