Add polarisation spin state to ISIS ORSO file metadata

[rbauststfc]

For POLREF data, we need to identify which spin state is being saved by populating the measurement->instrument_settings->polarization metadata in the header of the ISIS ORSO file that's produced by SaveISISReflectometryORSO.

We should enter the following in the ORSO file to identify the spin state:

• PNR (half-polarised): po or mo
• PA: pp, pm, mp or mm

Eventually there would be an ANR (with only the analyser) case, however we don't currently support ANR reduction in Mantid, so this is out of scope of this issue.

To identify the spin state of a workspace in the SaveISISReflectometryORSO algorithm, I think the most reliable approach would be for polarisation correction algorithms PolarizationCorrectionWildes and PolarizationCorrectionFredrikze to add an entry to the sample log that provides this information for each child workspace in the input group.

Our scientists have expressed a preference that we use the ORSO notation in our sample log. The two correction algorithms use different notation for parallel and anti-parallel:

• PolarizationCorrectionWildes: we should set the spin state in the sample log from the name of the output workspace. For example, the Wildes ++ output workspace would be spin state pp, +- would be pm, -+ would be mp and -- would be mm. It's important to set the spin state based on the output workspace name rather than the input flipper configuration as the Wildes flipper configuration does not tell us the spin state, it just tells us which flippers are on or off on the instrument. An instrument can be set up for cross-polarization, which would mean the flipper configurations correspond to different input spin states than they would normally. Regardless of the instrument setup, the maths in the calculation handles this automatically so that the final outputs are labelled with their correct spin state.
• PolarizationCorrectionFredrikze: subscripts pp, pa, ap, aa are used for the input measurements and corrected outputs. We need more information from our scientists about whether these can be used to identify spin state, or if different instrument setups will have an impact here.