Make output types of Vectors, Dyadics consistent.

This continues the change started by this commit:
375e12d139b8305c36bac5b8ca7637e0521b4ebc.
This commit also updates the affected doctests.
Fix physics/mechanics tests.

Fix physics/mechanics test to adhere to input restrictions as
implemented in the following commits:
375e12d139b8305c36bac5b8ca7637e0521b4ebc,
6902e1590471473d7559a0e88ffc4f7d5b2fcf3e,
5742b333ed893f4377e1c3315023e415338cb793.