Provides a floor gradient given the geometry of a scenario. This class uses the Fast Marching algorithm (Sethian 1999) to compute a solution to G(x)||grad T(x)|| = 1, the Eikonal equation. G(x) must not be equal to one, like in FloorGradientProviderContinuous, however, the solution of FloorGradientProviderDiscrete is only present on a discrete grid. CAREFUL: this destroys (= resets to 1) the convergence rates of many higher order integrators if mollification of the gradient is done improperly!