- 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_cast.cpp

@@ -0,0 +1,3 @@
+Matrix2d md = Matrix2d::identity() * 0.45;
+Matrix2f mf = Matrix2f::identity();
+cout << md + mf.cast<double>() << endl;