GUI: Change the units of blocks easily

As someone using IBEX, there are times when the default units do not actually relate to what I'm measuring (e.g. the use of Eurotherms to measure pressure), I'd like to be able to easily change the units associated with my measured values.

## Acceptance Criteria
- [ ] There is an option in block creation to override the default units
- [ ] There is an option in the IOC settings to override the default units

## Notes
- This should generally be possible by setting the appropriate field in the record, but a less "privileged knowledge" way of achieving it would be helpful to the instrument scientists