Add documentation and very simple test for array atan().

Eigen/src/plugins/ArrayCwiseUnaryOps.h

@@ -142,6 +142,9 @@ tan() const
 }
 
 /** \returns an expression of the coefficient-wise arc tan of *this.
+  *
+  * Example: \include Cwise_atan.cpp
+  * Output: \verbinclude Cwise_atan.out
   *
   * \sa cos(), sin(), tan()
   */