Improve RadMaps accuracy

sub-steps with fixed length (used to be 1 mm) could lead to inaccurate density calculations if the binning in r is getting small. This is mitigated by randomizing the map filling flat along the sub-step. At the same time this allows to increase the max sub-step length keeping running time approx. the same. For geantino scans a weight proportional to the radial distance from the IP is used in order to improve the volume fraction accuracy.