define position vector utils, default cartesian

Hi Burt and Ted,
   I have to deal with some detector geometric positions, and it would be useful to have position vectors available in the same way as lorentz vectors.
I also had to modify my linkdef, see [https://github.com/davidegerbaudo/supy-l1stgc/commit/355024d778cfbb21117b7a264c67396a9bcf0e8c#cpp/linkdef.cxx](this link). But that's outside supy.
Thanks,
Davide