Define non-const operator() in Reverse; enable test for this.

Introduction of DenseCoeffBase (revision bfdc1c4973
) meant that non-const
operator() is only defined if DirectAccess is set. This caused the line
"m.reverse()(1,0) = 4;" in MatrixBase_reverse.cpp to fail at compile-time.
Not sure this is correct solution; perhaps we should disallow this? Or make
Reverse DirectAccess with a negative stride - would that break something?

Eigen/src/Array/VectorwiseOp.h

@@ -380,8 +380,8 @@ template<typename ExpressionType, int Direction> class VectorwiseOp
     /** \returns a matrix expression
       * where each column (or row) are reversed.
       *
-      * Example: \include PartialRedux_reverse.cpp
-      * Output: \verbinclude PartialRedux_reverse.out
+      * Example: \include VectorWise_reverse.cpp
+      * Output: \verbinclude VectorWise_reverse.out
       *
       * \sa DenseBase::reverse() */
     const Reverse<ExpressionType, Direction> reverse() const