- rework the coefficients API

- make vectors use a separate loop unroller, so that copying a
row-vector into a col-vector is now possible
- add much more documentation
- misc improvements

doc/snippets/MatrixBase_block.cpp

@@ -0,0 +1,3 @@
+Matrix4d m = Matrix4d::diagonal(Vector4d(1,2,3,4));
+m.block<2, 2>(2, 0) = m.block<2, 2>(2, 2);
+cout << m << endl;