The multiple_scattering flag was required if absorption_corrections was true, despite multiple_scattering not being available.
This changes multiple scattering to be optional.

This is to match other isis_powder style functions (e.g. create_vanadium and focus)

Given the nature of it being post reduction analysis, it might be worth adding the ability to provide a workspace.
This would have to be discussed with an instrument scientist to see if it of use.

Refs #21294

Refs #21294

Merge to ensure CrystalFieldFunction is available and up to date.