Changed the definition of the rotate3_inertia function.

See the note in the commit.

Check out the regression test failure in test_human.py on line 851. This shows that the inertia we were previously computing is similar to the new values, but for some signs changes in products of inertia. If we agree that this is the correct fix we will need to update all of the regression tests that were based on the previously computed values.

- [ ] Change all of the regression tests that are failing such that they reflect the new inertia values.