Make ``PeriodicTrigger`` inherent from ``Trigger``
Make ``PeriodicTrigger`` inherent from ``Trigger``

Fixes bug in checking state and preprocessing ``Triggers`` for duck
typing.

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