ENH: allow specific restart time for field averaging (issue #282)

- Can currently have a periodic restart, but for simulations with a
  known run-up, it can be useful to have a specific time to restart
  the averaging.

- Note that the restartTime acts as a 'single-shot'. If the
  restartTime is already in the past when a simulation is started, it
  is ignored. If, during a simulation, the restartTime is crossed, it
  will be triggered and then set itself to be ignored in the future.