Keysight E4980AL LCR Meter: Provide nicer way of specifying function

Following on from #7663

Currently setting a function on the Keysight LCR meter is just done by a text box -> stringout setpoint. A user would be notified by the error ID if they put in an invalid function, but this really isn't ideal as they might do it in a script and forget to check the error id.

THe front panel of this device basically groups by the first measurement ie
Cp - D, G, etc
Lp - Rp etc

We should provide the scientists with something similar.

NB - there are ~22 functions that can be selected, so using an mbbi and calc isn't really an option.

### Acceptance criteria
- The IOC is modified to give a list of first measurements, and then set a list of second measurements depending on the first measurement.
- The OPI is modified to reflect this/the front panel of the actual device more closely