ENH: rationalize cloud method inheritance

- a top-level cloud::nParcels() virtual, which is overloaded by the
  first level of Cloud inheritance. This permits quick determination of
  cloud sizes, even when retrieved from registry with the base level.
  Eg,
      cloud* cldPtr = mesh.cfindObject<cloud>("myCloud");
      label nParcels = (cldPtr ? cldPtr->nParcels() : 0);

- make writeLagrangianPositions on by default unless explicitly
  disabled in the InfoSwitches.

  Flag output errors (where neither coordinates nor positions are
  written) with Fatal.

- additional IOField helper functions in cloud

STYLE: simplify iterator inheritance