EMU: Booster Heater improvements

As an EMU scientist, I would like some improvements to how I use the booster in IBEX:

- ON/OFF state block
- My users would like a block that displays whether we are using boost or not
- This block should monitor the Cryostat_Booster block and if it is NO_BOOSTER then it should be OFF else it should be ON
- I would like `r.boost(BoosterUseState)` and `r.noboost()` functions to set the booster state
- These will set the Cryostat_Booster block accordingly
- This script can be implemented by the scientist but they may need help with it

Acceptance criteria:

- PV for ON/OFF state block exists and updates correctly
- `r.boost` and `r.noboost` functions exist

Notes:

- This was requested for the next cycle in April 2021