(For MRPT 2.0) Make rnav fully compatible with simulated time

[jlblancoc]

Possible implementation, pass a C++11 chrono's clock which by default may be the real-time one, but could be replaced by a simulated one (as suggested elsewhere by @jolting ).

Code changes:

• timestamp_reach in waypoints nav class
• delays checks in abstract PTG rnav class:
  • port all delays-model to double and use robotTime() to make this compatible with faster-than-real-time simulators.
  • find usages of mrpt::system::timeDifference