AST-1295 Adapt NullStokes process methods to unique_ptr interface
Update NullStokes/tNullStokes to unique_ptr process interface Update NullStokes to xtensor Simplify NullStokes code in the process and make it more concise and easier to follow
Update NullStokes/tNullStokes to unique_ptr process interface Update NullStokes to xtensor Simplify NullStokes code in the process and make it more concise and easier to follow