fixed sized Eigen vectors to fix crash at startup

[enricocovili]

This pull request refer to #62 as stated in the last message

[LimHyungTae]

Solved by #81. PR is closed.

Thanks for raising the issue and pursuing the fix @enricocovili. We went with #72's approach (Vector3f for all three of normal_, singular_values_, pc_mean_) because singular_values_.minCoeff() is used as the flatness measure at cpp/patchworkpp/src/patchworkpp.cpp:221, which operates on the smallest of three singular values from JacobiSVD<MatrixX3f> — Vector2f would have silently changed ground segmentation behavior.