Throw an OutOfRange exception in SurfaceProcessor::setProbeRadius
Throw an OutOfRange exception in SurfaceProcessor::setProbeRadius

It was possible to set a radius <= 0. Now an OutOfRange excpetion is
thrown in these cases. This fixes bug #95

tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball