[ODE] Fixed stepping

[Geoff Carlton]

Hi,
I don't think its a good idea to fundamentally change the way all 
existing (correct) code works, which may well be implementing fixed 
stepping logic outside ODE.   Two ideas:

1.) Optional stepping via the same interface:
If the dWorldSetTimeStep is called, all solver functions do the 
additional fixed step logic loop, otherwise they behave as normal.

2.) A whole new interface, either one of:
a)
  dWorldSetFixedTimeStep(world, period)
  dWorldFixedStep(world, delta, steptype)
b)
  dWorldFixedStep(world, delta, period, steptype)

In the above functions, steptype is an enum, of which possible values 
could be:
  SOLVER_ORIGINAL
  SOLVER_QUICK
  SOLVER_FAST
  SOLVER_LATEST = SOLVER_FAST  (or whatever is latest)

Geoff