npsim gun shoudl have --gun.etaMin and --gun.etaMax

For simulations uniform over eta, it might make sense to have a parallel setting to

  --gun.thetaMin GUN.THETAMIN
  --gun.thetaMax GUN.THETAMAX

that allow one to set eta min and max:

  --gun.etaMin GUN.ETAMIN
  --gun.etaMax GUN.ETAMAX

It would need some way of figuring out what to do when both etaMin/Max and thetaMin/Max are passed.