Rho cutoff has to be introduced

There is a rho cutoff value in [Runafluid](https://github.com/osrep/Runafluid/blob/master/runafluid.cpp#L61) and [Runin](https://github.com/osrep/Runin/blob/master/runin.cpp#L22) to ignore the runaway population in the edge of the plasma. A similar method should be implemented to NORSE.