Improve Y-Axis Units in ISIS SANS Reduction and Load/SaveCanSAS

[Peter Parker]

#9710, #9721 and #9772 were all "band-aid" fixes, trying to get correct unit labels for the y-axis of ISIS SANS-reduced workspaces for the purposes of plotting, as well as writing out to CanSAS data files which are then loaded into SasView.

There are some problems that need to be addressed:

• SaveCanSAS looks for YUnitLabels of "I(q) (cm-1)" and writes out I and Idev units of "1/cm", else it writes out whatever the YUnitLabel is. Since the CanSAS format is strict about ​which units can be used, we need to get a better handle on what other units SaveCanSAS should be able to write out (if any), and what the YUnitLabels of the workspaces being written have to be in those cases. Should we disallow workspaces with YUnitLabels we don't recognise, or should we default to some sensible unit and output a warning?

• LoadCanSAS does not (and has never) read in y-axis unit labels, meaning YUnitLabels on plots do not survive the roundtrip of Save/LoadCanSAS.

The algorithm documentation should be updated accordingly.

[Stuart Campbell]

This ticket has been transferred to github issue ​10629