Refactor Fredrikze Algorithm SpinStates Implementation

- Renamed the input and output spin states
- refactored tests to eliminate code duplication
- added an helper function to fix crash which happens when user enter pp, aa as spinstates
- improved code based on PR review suggestions
- updated unit tests to handle crash which happens when user enter pp, aa as spinstates
- updated documentation

Framework/Algorithms/inc/MantidAlgorithms/PolarizationCorrectionFredrikze.h

@@ -47,7 +47,7 @@ class MANTID_ALGORITHMS_DLL PolarizationCorrectionFredrikze final : public API::
   std::shared_ptr<Mantid::API::MatrixWorkspace> add(const std::shared_ptr<Mantid::API::MatrixWorkspace> &lhsWS,
                                                     const double &rhs);
   std::shared_ptr<Mantid::API::MatrixWorkspace> multiply(const std::shared_ptr<Mantid::API::MatrixWorkspace> &lhsWS,
-                                                         double &rhs);
+                                                         const double &rhs);
 };
 
 } // namespace Algorithms