HIFIMAGS: allow setting persistent flag in all field modes

# Feature Description

As an inst. sci. on HIFI I would like to be able to set the HIFIMAGS persistent mode flag in all field modes.

## Explanation

> [...] as the button is only setting a flag to be acted on the next time the main magnetic field is set, I can’t see the harm in allowing setting it any time. In particular it would speed up going from low field mode (with the main magnet persistent at zero for better stability) to high field non persistent: changing mode re-sends zeros to many supplies and that would mean warming and re-cooling the switch before warming it again to set the LF value wanted for the next run.

# Acceptance Criteria

- Persistent mode flag settable regardless of current field mode
- Consequences of this explicitly discussed and understood with scientists

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