feature request: command to limit step size

[jasondet]

The method is to just do e.g.

G4UserLimits* userLimits = new G4UserLimits();
userLimits->SetMaxAllowedStep(stepSize);
logVol->SetUserLimits(userLimits);

It should therefore be pretty straightforward to add a command like /g4simple/setStepLimit [length] (volnameregex) where one loops over phys vol names (as we do in /g4simple/listPhysVols) and send a limit to the logical volume of any matches (or to all volumes if a vol name is not supplied).