Reflectometry: Footprint calculator POLREF requirements

As a reflectometry user, I would like the footprint calculator to work correctly on all reflectometers. E.g. on POLREF it is possible to run the reflectometer with only two active slits. Additionally, it needs the ability to change values for lambda min/max at runtime (since it changes with mode on POLREF)

#### Acceptance Criteria
- Calculator can sensibly deal with any configuration of slits on the beamline being enabled/disabled at runtime
- Can change lambda min/max used in footprint calculator at runtime
- `BeamlineConstant` can change on mode change
- TODO in config file is resolved
- Find out whether there are any additional requirements
- Can we close ISISComputingGroup/IBEX#5092?
- Check correctness of calculated numbers
- Read back whether sample is over or under illuminated

### Notes

If the slits are not there then they act as if they are wide open not giving any resolution