* add resize(int, NoChange) and resize(NoChange, int)

* add missing assert in resize(int)
* add examples for all resize variants
* expand docs (part of which is from Tim Hutt's e-mail)

doc/snippets/Matrix_resize_int_int.cpp

@@ -0,0 +1,9 @@
+MatrixXd m(2,3);
+m << 1,2,3,4,5,6;
+cout << "here's the 2x3 matrix m:" << endl << m << endl;
+cout << "let's resize m to 3x2. This is a conservative resizing because 2*3==3*2." << endl;
+m.resize(3,2);
+cout << "here's the 3x2 matrix m:" << endl << m << endl;
+cout << "now let's resize m to size 2x2. This is NOT a conservative resizing, so it becomes uninitialized:" << endl;
+m.resize(2,2);
+cout << m << endl;