Counter rotating choppers

[nvaytet]

Fix #5

Note that the definition of frequency has changed slightly:

• Say we define two chopper openings with positive angles (meaning anti-clockwise direction), one opening starting at 10 deg, and the second starting at 90 deg.
• The chopper needs to be rotating in the clockwise direction if the opening starting at 10deg is to be the first one that crosses the beam.
• Therefore, the chopper should have a negative frequency,

This implies that all the frequencies in the tests and docs have now been converted to negative frequencies.

This also means that all users have to change the sign of their frequencies :-(

[nvaytet]

Implemented in #47 instead