Clean up force application code.

[Rick Stovall]

In cpp/sim_engine/base_object/base_object.cpp:

bool base_object::apply(int time, float quanta)
{
 (....)

  // existing code (too ugly to live)
  velocity += (a * quanta) + (accel_mod * quanta);
  rotation += (ra * quanta) + (torque_mod * quanta);

  // should be
  velocity += (a + accel_mod) * quanta;
  rotation += (ra + torque_mod) * quanta;

(....)
};

This new code will be easier to understand and may run better.