- 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_row.cpp

@@ -0,0 +1,3 @@
+Matrix3d m = Matrix3d::identity();
+m.row(1) = Vector3d(4,5,6);
+cout << m << endl;