Generic expression of a partially reduxed matrix.
| MatrixType | the type of the matrix we are applying the redux operation | 
| MemberOp | type of the member functor | 
| Direction | indicates the direction of the redux (Vertical or Horizontal) | 
This class represents an expression of a partial redux operator of a matrix. It is the return type of some VectorwiseOp functions, and most of the time this is the only way it is used.
Inherits internal::dense_xpr_base::type, and Eigen::internal::no_assignment_operator.
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    https://eigen.tuxfamily.org/dox/classEigen_1_1PartialReduxExpr.html