Eigen's Matrix. Now the dilema is how to mimic those typedefs, using inheritence ?
or using the classic workaround: typename Point3<float>::Type; with Point3<T>::Type defined
to Eigen::Matrix<T,3,1>. Anyway currently I support both (and the inheritence scheme has
to be preserved for compatibility). The advantage of the second approach is that when
eigen has to evaluate an expression it uses an Eigen::Matrix<>, so it is probably better
to only use Eigen::Matrix but I'm not 100% sure that makes a big difference especially if
we add some automatic reinterpret_cast between Eigen::Matrix and vcg::Point*....
