ConstantVelocity integration method

[joaander]

Description

The ConstantVelocity integration method would integrate particle positions forward in time at a constant velocity, ignoring all forces applied to the particles. It would also integrate orientations forward at constant angular velocity.

Proposed solution

The solution would make use of ConstantVolume at the C++ level, with a flag to always zero the acceleration and torque. At the Python level, we would expose it as ConstantVelocity(filter). Users would set the velocities of the particles in the simulation state.

Additional context

HOOMD v2 had this in the zero_force option to NVE. It is useful to create flows of particles (i.e moving walls). It can also be used in 2 particle systems to measure pair potentials in one call to run.

This should be implemented on trunk-major to avoid merge conflicts with the refactored integrators.