LOQ: Set shutter to closed when count rate is too high

As an instrument scientist I want to prevent damage to my detector from a neutron flux it was not designed for. When the count rate is above a defined maximum I want the aperture to move to block the path of the beam

The issue names no files, tests, or entry points; start by locating the count-rate handling and aperture or shutter control in the IBEX repository. Clarify the defined maximum and verify that the aperture moves to block the beam whenever the count rate exceeds it.

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