DiskChopper, rotation seems to be left-handed with positive frequency

[willend]

Issue raised in connection with this mcstas-users thread:
https://mailman2.mcstas.org/pipermail/mcstas-users/2021q2/001420.html

As suggested in the thread, we should:

1. Fix the documentation so that it is clear that the default “sign” of rotation is left-handed
2. Fix mcdisplay so that it reflects what the neutrons will “see” at t=0
3. Potentially add a sign-parameter to choose the “handedness”