Maintenance tasks for PolarizationCorrectionFredrikze

[rbauststfc]

Following on from #37672 we have identified the following maintenance that would be good to do for algorithm PolarizationCorrectionFredrikze:

• Move the PNR_LABEL and PA_LABEL constants out of the Prop namespace and into the unnamed namespace below that holds all of the other label constants.
• For the constants in the unnamed namespace, all should be of type static const std::string. The names should all be upper case with words separated by an underscore, so crhoLabel would become CRHO_LABEL, cppLabel would become CPP_LABEL, etc.
• Where we create the spinStateValidator by calling std::make_shared<SpinStateValidator>(std::unordered_set<int>{2, 4}, true, 'p', 'a', true); we should replace 'p' and 'a' with the constants SpinStateConfigurationsFredrikze::PARA and SpinStateConfigurationsFredrikze::ANTI.
• In the algorithm documentation, the DOI in the References section at the bottom should be a hyperlink, but it doesn't seem to be rendering correctly.
• In the algorithm documentation under the Spin State Configurations section, the note that gives the defaults of the InputSpinStateOrder property should probably be removed and that information should be integrated into the bullet points above. At the moment having a separate note makes the layout a bit confusing.

This final task should not be done until we have completed work on #38265:

• We should make sure that existing unit tests are checking that the re-ordering of workspaces when different input and output spin states are passed in is being applied correctly. At the moment we seem to be assuming that as long as the algorithm completes without error then everything is OK. It may be that we can do this most effectively for the output spin state order after Add output spin state to sample log in PolarizationCorrectionFredrikze #38265. (Addressed as part of PR Add Output SpinState to SampleLog in PolarizationCorrectionFredrikze Algorithm #38447)