ALARMS: set alarm levels based on offset or percentage

On RIKENFE (and perhaps elsewhere) it would be useful if the alarm levels
for PVs could be defined as a percentage of the setpoint, or as the setpoint
+- some absolute difference.

At present, the beamline tuning script sets the alarms to match. But with
cset() or set_pv() or using the OPI GUI, the alarm level are unchanged and
there may be an alarm if the user intentionally tunes something.

Notes:

The underlying EPICS behaviour only supports absolute values for HIGH, LOW etc. so we would need to add a mechanism ourselves, possibly something like run control (so you enable “automatic alarm adjustment” and a threshold value and then some underlying logic would monitor changes in SP and adjust alarm levels accordingly).