Rhs chopper

[ggoneiESS]

The handedness of the NeXus coordinate system implies that chopper rotation should be positive when rotating clockwise facing away from the source, but the documentation states it is the exact opposite.

This cannot be corrected easily historically, as many systems should have this in place, but it is not correct to describe NeXus as using a right-handed coordinate system with this discrepancy. It also leads to an unintuitive and ugly exception when describing an instrument.

This PR proposes a fix by including a new field, handedness, that defaults to Left unless otherwise specified, thus not breaking any existing setups or data files, but allows for another value, Right, which agrees with the standard NeXus coordinate basis and which arises naturally when defining a RHS with a z-axis that runs in a positive direction from source to sample.