OFFSPEC: map B-H hysteresis curves

## Where?

As an instrument scientist on OFFSPEC I would like to map out B-H hysteresis curves for my magnets. (B is magnetic flux density in e.g. `Teslas`, H is external applied magnetisation force in e.g. `A/m`).

To do this in SECI the scientists use a VI located in `c:\labview modules\instruments\offspec\offspec bh\offspec bh - system functions.llb\offspec bh - front panel.vi`.

In IBEX the planned implementation is to use our existing standalone KEPCO + Keithley 2400 drivers, plus a small script which uses the scans library to reproduce the functionality of this VI. This ticket is to write the script.

## Acceptance criteria
- [ ] As an instrument scientist on OFFSPEC, I can map out B-H hysteresis curves for magnets. This should be *functionally* equivalent to the OFFSPEC BH vi linked above, but can be driven from a script rather than GUI

## Notes
- The scientists do this "every cycle or so" - so I understand this is a requirement for initial OFFSPEC migration.

## How to Test
_verbose instructions for reviewer to test changes
(**Add before making a PR**)_