Runtime limit and resubmission command

Implements #461 (closed).

We create a new parameter to set the maximal wall-clock time of a given run. When the time is reached the code dumps a restart file and exits. An additional set of parameters allow the user to specify (or not) a command to be run as SWIFT exits. This is, for instance, a convenient way of having the code re-submit itself to the batch queue and resume the run without human intervention.

This uses the system() command and makes no check on what the user specified as a command. Do you see a better way of achieving this?