Use Eigen instead of Wm3 for math without disrupting source code

This will make it possible to get rid of wm3 and use eigen instead, without touching bulk of the code. Things to handle:

1. define what we need (Vector3, Vector2, Quaternion, Matrix3; that should be all)
2. write the wrapper itself (out of trunk, probably in lp:~yade-dev/yade/snippets or create separate branch for that)
3. write tests and make sure results are the same if run on wm3 or eigen (still out of trunk)
4. update python wrapper around wm3 (will most likely not work as-is, since autogenerated)
5. update serialization code, if needed
6. test again
(7. remove the wrapper, changing the code in an automatized way; not strainghtforward, might never happen as well)