Fix bug: ghost particles not always available
Fix bug: ghost particles not always available

SFCPackUpdater ran a migrate but not exchange. This caused ghost particles to disappear for later stages in the computation.
joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue