Write page on template and typename keywords in C++.

After yet another question on the forum, I decided to write something on this
common issue. Now we just need to link to this and get people to read it.
Thanks to mattb on the forum for some links. Caveat: IANALL (I am not a
language lawyer).

doc/examples/TemplateKeyword_flexible.cpp

@@ -0,0 +1,22 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+
+template <typename Derived1, typename Derived2>
+void copyUpperTriangularPart(MatrixBase<Derived1>& dst, const MatrixBase<Derived2>& src)
+{
+  /* Note the 'template' keywords in the following line! */
+  dst.template triangularView<Upper>() = src.template triangularView<Upper>();
+}
+
+int main()
+{
+  MatrixXi m1 = MatrixXi::Ones(5,5);
+  MatrixXi m2 = MatrixXi::Random(4,4);
+  std::cout << "m2 before copy:" << std::endl;
+  std::cout << m2 << std::endl << std::endl;
+  copyUpperTriangularPart(m2, m1.topLeftCorner(4,4));
+  std::cout << "m2 after copy:" << std::endl;
+  std::cout << m2 << std::endl << std::endl;
+}

doc/examples/TemplateKeyword_simple.cpp

@@ -0,0 +1,20 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+
+void copyUpperTriangularPart(MatrixXf& dst, const MatrixXf& src)
+{
+  dst.triangularView<Upper>() = src.triangularView<Upper>();
+}
+
+int main()
+{
+  MatrixXf m1 = MatrixXf::Ones(4,4);
+  MatrixXf m2 = MatrixXf::Random(4,4);
+  std::cout << "m2 before copy:" << std::endl;
+  std::cout << m2 << std::endl << std::endl;
+  copyUpperTriangularPart(m2, m1);
+  std::cout << "m2 after copy:" << std::endl;
+  std::cout << m2 << std::endl << std::endl;
+}