Check / prove that integrator test is valid for temperature / pressure coupling

[ptmerz]

The integrator test analyzes the fluctuations of the constant of motion of a simulation. Theoretically, this is expected to be proportional to dt^2 (since the integrator preserves the constant of motion of the shadow Hamiltonian, but we are observing the constant of motion of the real Hamiltonian...).

We are only advertising to use this in NVE simulations, since we never cleanly proved that it would work with constants of motion of extended systems using temperature or pressure coupling. It shouldn't be too hard to check this & update the documentation accordingly.

[ptmerz]

Bumped since this isn't a blocking issue.