Big renaming:

start ---> head
  end   ---> tail
Much frustration with sed syntax. Need to learn perl some day.

doc/snippets/MatrixBase_start_int.cpp

@@ -1,5 +1,5 @@
 RowVector4i v = RowVector4i::Random();
 cout << "Here is the vector v:" << endl << v << endl;
-cout << "Here is v.start(2):" << endl << v.start(2) << endl;
-v.start(2).setZero();
+cout << "Here is v.head(2):" << endl << v.head(2) << endl;
+v.head(2).setZero();
 cout << "Now the vector v is:" << endl << v << endl;