First step of a big rework of Eigen to make the dynamic-size and fixed-size classes share common code, using a C++ trick known as Curiously Recurring Template Pattern, as described here: <a href="http://www.informit.com/articles/article.asp?p=31473&seqNum=3&rl=1">http://www.informit.com/articles/article.asp?p=31473&seqNum=3&rl=1</a>;
Thanks go to the gurus on the kde-devel list, especially Sylvain Joyeux.

This patch adds a new class template VectorBase, and reimplements the dynamic-size VectorX as a derived class template of VectorBase.

TODO: reimplement in the same way all the other classes.