DANFYSIK: set high/low readback limits automatically

As a muon scientist I would like setting CURR:SP on a danfysik to also set CURR.LOW and CURR.HIGH to appropriate values when the SP is updated. This logic may need to allow for both a % or fixed tolerance - Johnny will check with muon group.

Probably best to create a utility calc record, a bit like unit setter, that can be used for this sort of thing more generally and allow for %/fixed/choice

Should make enabling this conditional to not upset any other people already using a script to explicitly set LOW/HIGH - so any default values would need to be "do nothing"

## Acceptance criteria

* LOW and HIGH limits set appropriately