remove USING_PART_OF_NAMESPACE_EIGEN, leaving it in Eigen2Support.

improve porting-Eigen2-to-3 docs

disabled/class_Eval.cpp

@@ -1,5 +1,5 @@
 #include <Eigen/Core>
-USING_PART_OF_NAMESPACE_EIGEN
+using namespace Eigen;
 using namespace std;
 
 template<typename Derived>

disabled/ompbenchmark.cpp

@@ -9,7 +9,7 @@
 // #include "BenchTimer.h"
 //
 // using namespace std;
-// USING_PART_OF_NAMESPACE_EIGEN
+// using namespace Eigen;
 //
 // enum {LazyEval, EarlyEval, OmpEval};
 //