Fixed a bug that appeared when running the uniaxial-post.py script from ...

Fixed a bug that appeared when running the uniaxial-post.py script from examples. Edited py/post2d.py to capture exception while importing Vector3 from minieigen.

I guess though the statements might not be needed anymore as the imports have already been done in the beginning of the file. 

I am new to contributing to your repos so please inform me if there are any special procedures I should follow.