Protect against extrapolations to large z values

I added a protection against large z values into the DistanceCalculator and the VertexFitter. In the DistanceCalculator this is a property. For the VertexFitter it is done in the transport method of LoKi::KalmanFilter without a property. I could put it into the VertexFitter as well. If there are better suggestions to implement it, this is welcome. @rmatev, @ibelyaev