Resolve "Fix the documentation block of the variable OEPMixingScheme"

Description

Fix the documentation block of the variable OEPMixingScheme

News snippet

Fix the documentation block of the variable OEPMixingScheme

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #604 (closed)