"element" doesn't work with angle

[Laurie Nevay]

`angle` has wrong sign convention as compared to other angled components (rbend, sbend). This is because of MADX’s convention that has an arbitrary flip in x.

We don’t adapt the incoming / outgoing drifts to the angle of the geometry or provide no way to do this.

The length supplied is treated as the arc length but in reality it can only really be the chord length given choices of container solids in Geant4 for the single GDML file we place without offset.

[Laurie Nevay]

Fixed this.

• -1 introduced for angle.
• element now accepts e1 and e2, which aren't used for construction but drifts before and after it will now obey.
• the l length parameter is now treated as the chord length by default. The parameter elementLengthIsArcLength is available to interpret it as arc length if required but is off by default.