[PotentialFieldModel] Replace Cartesian grid by unstructured mesh
In some scenarios (e.g., RIMEA 10 for GNM grid resolution = 0.1), agents get stuck at walls if Cartesian mesh is too coarse.
In some scenarios (e.g., RIMEA 10 for GNM grid resolution = 0.1), agents get stuck at walls if Cartesian mesh is too coarse.