big improvement of the block-manipulation API

- reduction of sizeof(Block) for vector types
- variants of block() and fixedBlock() for vector blocks
- convenience methods start() and end() for vectors
- convenience method corner() for matrices

doc/snippets/MatrixBase_block_int_int_int_int.cpp

@@ -0,0 +1,5 @@
+Matrix3d m = Vector3d(1,2,3).asDiagonal();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here is m.block(1, 1, 2, 1):" << endl << m.block(1, 1, 2, 1) << endl;
+m.block(1, 0, 2, 1) = m.block(1, 1, 2, 1);
+cout << "Now the matrix m is:" << endl << m << endl;