3D magnet: eliminate quad-state booleans

As a scientist and developer using the 3D cryomag on zoom, I would like to be able to change the status of the switch heaters reliably.

This driver is an LvDCOM driver, but it appears that at some point the converter has become confused and generated multiple strings for the same item(s). See https://github.com/ISISComputingGroup/EPICS-Magnet3D/blob/698116ffd43389b413226bb998e3b37902829fcc/magnet3DApp/protocol/controls.txt#L931 for an example

```

OFF
ON
OFF
ON

```

@FreddieAkeroyd and I had a look at this and suspect that this was causing some support calls on zoom related to not being able to change the state of the switch heater intermittently. On examining the underlying labview code, it appears that the relevant code is a simple boolean, so presumably cannot be in 4 states.

Acceptance criteria:
- Issue in 3D magnet is fixed
- Other similar issues in different devices and looked for and corrected if necessary
- The LvDCOM converter scripts warn about this case when first doing a conversion.