v4.1.0

• many bugfixes
• support for basic dynamics: position models simulate acceleration
  and have a new acceleration control mode.
• visualization options can be loaded from and saved to worldfiles
• API change: new World::Run() methed provides the same interface to
  GUI and non-GUI worlds (by chmp@github)
• saved screenshots are numbered with leading zeros for convenience
  when using movie-making tools (by mani-monaj@github)
• major performance enhancement: position updates now run in parallel
  with sensor updates, using a double-buffer-style alternating pair of
  occupancy grids
• several minor performance enhancements
• much faster debug visualization (occupancy grid and voxel views)
• build scripts are more portable