EMU: zero field setup: requested changes

As an EMU instrument scientist I would like the following (non-urgent) changes to the zero field setup routine:

### Acceptance Criteria

- All 3 magnetometer axes should be plotted on each plot of current against measured field. This ensures that there isn't excessive crossover between the axes (one axis should respond, the others should be mostly flat on each plot)
- The zero field setup routine should only calculate the line of best fit based on the "central region" of each plot. Each plot is 21 points, so an example would be to calculate the line of best fit only on the middle 11 points (this was discussed and agreed with SC, KY, JL). All of the points should still appear on the graph.
- When doing the calibration, read the existing calibration factors and warn if they are different to the new ones by more than 10%

### Notes

Should be done with instrument scientists if possible so that they gain ownership of the scripts.