Brownian dynamics: fix also disables rotation

[christophlohrmann]

MNWE:

from espressomd import System
import numpy as np

system = System(box_l=3 * [1.])
system.time_step = 0.01
system.cell_system.skin = 1
part = system.part.add(pos=3 * [0.], rotation=3 * [True], ext_torque=[0, 0, 1])

system.thermostat.set_brownian(kT=0., gamma=1., gamma_rotation=1., seed=41)
system.integrator.set_brownian_dynamics()

system.integrator.run(10)
np.testing.assert_allclose(part.omega_lab, [0, 0, 1], atol=1e-14)

part.fix = 3 * [True]
system.integrator.run(10)
np.testing.assert_allclose(part.omega_lab, [0, 0, 1], atol=1e-14)

Langevin/VV integrator works as expected

[christophlohrmann]

found the error, PR coming soon