add a VectorBlock expr as a specialization of Block

doc/examples/class_VectorBlock.cpp

@@ -0,0 +1,26 @@
+#include <Eigen/Core>
+USING_PART_OF_NAMESPACE_EIGEN
+using namespace std;
+
+template<typename Derived>
+Eigen::VectorBlock<Derived>
+segmentFromRange(MatrixBase<Derived>& v, int start, int end)
+{
+  return Eigen::VectorBlock<Derived>(v.derived(), start, end-start);
+}
+
+template<typename Derived>
+const Eigen::VectorBlock<Derived>
+segmentFromRange(const MatrixBase<Derived>& v, int start, int end)
+{
+  return Eigen::VectorBlock<Derived>(v.derived(), start, end-start);
+}
+
+int main(int, char**)
+{
+  Matrix<int,1,6> v; v << 1,2,3,4,5,6;
+  cout << segmentFromRange(2*v, 2, 4) << endl; // calls the const version
+  segmentFromRange(v, 1, 3) *= 5;              // calls the non-const version
+  cout << "Now the vector v is:" << endl << v << endl;
+  return 0;
+}