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_fixedBlock_int_int.cpp

@@ -0,0 +1,5 @@
+Matrix4d m = Vector4d(1,2,3,4).asDiagonal();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here is m.fixedBlock<2, 2>(2, 2):" << endl << m.fixedBlock<2, 2>(2, 2) << endl;
+m.fixedBlock<2, 2>(2, 0) = m.fixedBlock<2, 2>(2, 2);
+cout << "Now the matrix m is:" << endl << m << endl;