Various documentation updates:

- update the tutorial
- update doc of deprecated cwise function
- update cwise doc snippets

doc/snippets/MatrixBase_array_const.cpp

@@ -0,0 +1,4 @@
+Vector3d v(-1,2,-3);
+cout << "the absolute values:" << endl << v.array().abs() << endl;
+cout << "the absolute values plus one:" << endl << v.array().abs()+1 << endl;
+cout << "sum of the squares: " << v.array().square().sum() << endl;