fix: 4-argument TGeoTrap needs an atan

The [TGeoTrap class](https://root.cern.ch/doc/master/classTGeoTrap.html#a0e27ff95f2ffa2eb8c320a7778e1f403) (for which an object is created here) has a slightly different calling syntax than what is done internally in [G4Trap](https://apc.u-paris.fr/~franco/g4doxy/html/classG4Trap.html#074ca31710e0a11f9c7b6b13ddd1eabf). We need to take the atan here to get to the angle that TGeoTrap wants.