Allow 0 frequency choppers

[nvaytet]

Before these changes, creating a chopper which is parked with 0 frequency

tof.Chopper(
    frequency=0 * Hz,
    open=sc.array(dims=['cutout'], values=[30.0], unit='deg'),
    close=sc.array(dims=['cutout'], values=[140.0], unit='deg'),
    phase=0 * deg,
    distance=22 * meter,
    name="Pulse-overlap",
)

raised Chopper frequency must be positive.

Now, this is allowed. The chopper.open_close_times() returns and infinite range [-np.inf, np.inf], and the chopper blocks no neutrons.

[SimonHeybrock]

I think this is fine, if we assume that stopped choppers are parked on "open" position? If they are not, no neutrons would pass through. Thus doing a simulation would not be useful... except for diagnostics, but maybe that is secondary?

[nvaytet]

If they are not, no neutrons would pass through. Thus doing a simulation would not be useful.

Yes. They would just be removed from the simulation.
I believe choppers are almost always parked in the open position.

If someone really wants to park a chopper in closed position, they can give it a frequency of 1.0e-30 Hz or something like that.