Fix bug in creation of rigid bodies
Fix bug in creation of rigid bodies

The code always assigned the first N_body particles (by tag) to the be
the N_body centers. This fixes that error. This also fixes the
assignment of new constituent particles to the correct central body.

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