Fix hoomd.md.Integrator detaching.
Fix hoomd.md.Integrator detaching.

The method simply did not exist beforehand as an oversight. Now the
detaching happens appropriately.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue