Add converting between Chopper, DiskChopper and NeXus chopper descriptions

[nvaytet]

Examples:

From scippneutron.DiskChopper

disk_chopper = DiskChopper(
    axle_position=sc.vector([0.0, 0.0, 2.0], unit='m'),
    frequency=sc.scalar(12.0, unit='Hz'),
    beam_position=sc.scalar(45.0, unit='deg'),
    phase=sc.scalar(20.0, unit='deg'),
    slit_begin=sc.array(dims=['slit'], values=[0.0, 124.0], unit='deg'),
    slit_end=sc.array(dims=['slit'], values=[60.0, 126.0], unit='deg'),
)

chopper = tof.Chopper.from_diskchopper(disk_chopper)

To DiskChopper

chopper = tof.Chopper(
    frequency=12.0 * Hz,
    open=sc.array(dims=['cutout'], values=[-15.0, 45.0], unit='deg'),
    close=sc.array(dims=['cutout'], values=[15.0, 75.0], unit='deg'),
    phase=20.0 * deg,
    distance=2.0 * meter,
    name='chopper',
    direction=tof.AntiClockwise,
)

disk_chopper = chopper.to_diskchopper()

From NeXus

nexus_chopper = {
    'position': sc.vector([0.0, 0.0, 2.0], unit='m'),
    'rotation_speed': sc.scalar(12.0, unit='Hz'),
    'beam_position': sc.scalar(45.0, unit='deg'),
    'phase': sc.scalar(20.0, unit='deg'),
    'slit_edges': sc.array(
        dims=['slit'],
        values=[0.0, 60.0, 124.0, 126.0],
        unit='deg',
    ),
    'slit_height': sc.scalar(0.4, unit='m'),
    'radius': sc.scalar(0.5, unit='m'),
}

chopper = tof.Chopper.from_nexus(nexus_chopper)

Fixes #6

[jokasimr]

Looks good!