Truncate flux to 0

When there is no beam the flux PV can read as negative (https://ispyb.diamond.ac.uk/dc/visit/cm37235-2/id/13451673). This is potentially confusing for users and not immediately obvious that there is no beam. We should create two signals on the device, raw flux and then a flux that just reads 0 if the raw flux is very small or negative. Hyperion should then use the 0 reading one anytime it needs to read flux.

## Acceptance Criteria
* There is a signal on the flux device that reliably reads zero when there is no beam
* This signal is used in Hyperion when reading flux