ENH: add selectable update control/interval to pimpleFoam, rhoPimpleFoam

- Allows user-defined control of when the mesh motion occurs,
  which can be especially useful in situations where the mesh motion
  is much slower than any of the fluid physics.

  For example, in constant/dynamicMeshDict:

      updateControl   runTime;
      updateInterval  0.5;

  to have mesh motion triggered every 1/2 second.

  Note that the _exact_ time that the mesh motion actually occurs may
  be slightly differently since the "runTime" triggering is fuzzy in
  nature. It will trigger when the threshold has been crossed, which
  will depend on the current time-step size.