Virtual sites can have non-zero kinetic energy

[jngrad]

The translational and rotational kinetic energy kernels have diverged from one another in 4.2.0, such that a virtual site can have a non-zero rotational inertia and thus contribute a non-zero kinetic energy when spinning.

MWE:

import espressomd
assert espressomd.has_features(["MASS"])
assert espressomd.has_features(["ROTATIONAL_INERTIA"])
system = espressomd.System(box_l=[1, 1, 1])
def calc_kinetic_energy(**kwargs):
    system.part.clear()
    system.part.add(pos=[0, 0, 0], rotation=3 * [True], **kwargs)
    print(f"Particle(**{kwargs=}) -> {system.analysis.energy()['kinetic']}")
calc_kinetic_energy(v=[1, 2, 3], virtual=False)
calc_kinetic_energy(v=[1, 2, 3], virtual=True)
calc_kinetic_energy(omega_lab=[1, 2, 3], virtual=False)
calc_kinetic_energy(omega_lab=[1, 2, 3], virtual=True)

Output:

Particle(**kwargs={'v': [1, 2, 3], 'virtual': False}) -> 7.0
Particle(**kwargs={'v': [1, 2, 3], 'virtual': True}) -> 0.0
Particle(**kwargs={'omega_lab': [1, 2, 3], 'virtual': False}) -> 7.0
Particle(**kwargs={'omega_lab': [1, 2, 3], 'virtual': True}) -> 7.0

Expected output: the last line should have zero energy.