Add check to make sure the oscillations of the hydrostatic model are actually sinusoidal.

[arjo129]

The TEST(Stability, RestoringMoment) unit test only checks the maximum pitch but as @chapulina pointed out:

Should we check minPitch too?

It would also be interesting to check that the vehicle is actually bouncing up and down, because the current test would also pass if the amplitude is decreasing (which shouldn't happen because we don't have hydrodynamics loaded), or even if the vehicle were stopped in place. I'm not sure what the quickest way of testing this would be, maybe checking that the pitch rate keeps going up and down?

Original thread here: https://github.com/osrf/lrauv/pull/89#discussion_r773583989

We should try to find a way to check for oscillations.