bug #1071: improve doc on lpNorm and add example for some operator norms

doc/examples/Tutorial_ReductionsVisitorsBroadcasting_reductions_operatornorm.cpp

@@ -0,0 +1,18 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+using namespace std;
+
+int main()
+{
+  MatrixXf m(2,2);
+  m << 1,-2,
+       -3,4;
+
+  cout << "1-norm(m)     = " << m.cwiseAbs().colwise().sum().maxCoeff()
+       << " == "             << m.colwise().lpNorm<1>().maxCoeff() << endl;
+
+  cout << "infty-norm(m) = " << m.cwiseAbs().rowwise().sum().maxCoeff()
+       << " == "             << m.rowwise().lpNorm<1>().maxCoeff() << endl;
+}