POLREF: Redefine motors to be 0 on beam

As a scientist/supporter, I would like an axis position of 0 to consistently have the same meaning on reflectometry beamlines, that being that an axis at 0 is centered on the natural beam (unchanged by mirrors etc.). At the moment, there are some components on POLREF that have an offset so that 0 is at the parked position as that is how it was configured in SECI.

#### Acceptance Criteria
- [ ] Set offset on polariser flipper height to zero it on beam
- [ ] Set offset on Analyser 1 to zero it on beam
- [ ] Set offset on BOA to zero on beam (? pending scientist reply)
- [ ] Check CS parked position is correct

No files, tests, or entry points are identified in the issue. Start by locating the POLREF motor and beamline configuration, then confirm the BOA offset with a scientist; done means the polariser flipper, Analyser 1, and BOA are zeroed on beam and the CS parked position is correct.

Scritto dal modello di indicizzazione a partire dal testo della issue.