Scaling of single precision vector types with double precision factor

Problem: as in http://discourse.mcneel.com/t/small-probelm-with-python-rhinocommon-vector3f/5561
Cause: Python does not have single precision data type.
Solution: Add double precision scaling factor to single precision vectors.

PS. I see that the pull request contains also the example I wrote last year. I know that you think that should go elsewhere, but I'm not sure how to remove it from the pull request. Please ignore the example if at all possible.