Allow to create choppers from centers and widths of cutouts

[nvaytet]

Allow to create choppers from centers and widths of cutouts in addition to open and close angles.
Choppers are defined in terms of slit centers and widths in McStas.

Example:

tof.Chopper(
        frequency=14*Hz,
        direction=tof.AntiClockwise,
        width=sc.array(
            dims=['cutout'],
            values=[2.46, 3.02, 3.27, 3.27, 5.02, 3.93, 3.93, 2.46],
            unit='deg',
        ),
        center=sc.array(
            dims=['cutout'],
            values=[0, 72, 86.4, 115.2, 172.8, 273.6, 288.0, 302.4],
            unit='deg',
        ),
        phase=(286-180)*deg,
        distance= 6.145*meter, 
        name="PSC1",
    )