Genie python: Setting mbbi with float value causes the mbbi to be set to the zeroth value

As a user/scientist I would like to be able to set an mbbi record using a float value e.g. if we have an mbbi of (Off, 31.6uA, 100mA) when I call g.set_pv(mbbi, 0.0) it is set to Off, g.set_pv(mbbi, 1.0) it is set to 31.6uA and g.set_pv(mbbi, 2.0) it is set to 100mA.

This was the underlying bug when running r.settemp(0.2) on MuSR as we retrieved a value for the heater range (mbbi) of 3.0 from a tpar file and tried to set it. The current behaviour is to set the mbbi to the zeroth value, no matter what value has been set.

Acceptance criteria:

- [ ] A reasonable behaviour is agreed upon when setting mbbis using floats
- This behaviour will have to handle cases where we have a value that doesn't make sense e.g. 2.5
- [ ] The behaviour is implemented