Changed physics simulation step logic and made simulatedDevices/SimulatedDeviceROS parent tags optional.
Physics:
 * Changed stepSimulation call to be framerate-independent. Two parameters are parsed in the config file to tweak it:
   - physicsFrequency: simulation rate (default is 60, as in Bullet)
   - physicsSubSteps: in case the framerate is lower than 60, a max number of physics calculation steps to be done
       0 (default) for automatic number of substeps, i.e. ceil (elapsed/(1/physicsFrequency))
   For example if physicsSubSteps is set to 10 and physicsFrequency is 60, physics will be correct if FPS is higher than 6 (60/10).

Simuated devices:
  * Changed cirs.xml and UWSimScene.dtd to showcase both types of simulated device configuration options.