Fix chopper open and close times when direction is anticlockwise

[nvaytet]

There was a strange effect for anticlockwise choppers where when requesting more rotations, open and close times would just go more and more negative, instead of going the other way.
This PR fixes this.

Example:

import scipp as sc
import tof

Hz = sc.Unit('Hz')
deg = sc.Unit('deg')
meter = sc.Unit('m')
sec = sc.Unit('s')

chopper = tof.Chopper(
    frequency=10.0 * Hz,
    open=sc.array(dims=['cutout'], values=[10.0, 90.0], unit='deg'),
    close=sc.array(dims=['cutout'], values=[20.0, 130.0], unit='deg'),
    distance=10.0 * meter,
    phase=0 * deg,
    direction=tof.AntiClockwise,
    
)

print(chopper.open_close_times(0.0 * sec)[0].values)  # 2 rotations
print(chopper.open_close_times(0.2 * sec)[0].values)  # 3 rotations

Before:

array([0.06388889, 0.09444444, 0.16388889, 0.19444444])
array([-0.03611111, -0.00555556,  0.06388889,  0.09444444,  0.16388889, 0.19444444])

After:

array([-0.03611111, -0.00555556,  0.06388889,  0.09444444])
array([-0.03611111, -0.00555556,  0.06388889,  0.09444444,  0.16388889, 0.19444444])

[SimonHeybrock]

Not sure I follow 100% but the test seems to make sense.