Generate random run ID, track restarts

Improve data provenance tracking, so that sets of runs can be linked
to each other, and the run used to generate data identified.

Each time the solver is run, generate a random number which is
broadcast to all processors. This is stored as "run_id" in:
- log files
- BOUT.settings, though at the moment only if the run finishes
- restart files
- dump files

When a simulation is restarted, the ID of the run it started from is
also recorded as "run_restart_from".

If restart files are used which don't have a run_id, then this will
either cause the run to fail, or set run_id to 0. The
`run_restart_from` ID therefore has two special values:
- 1 means no restart, the run was started from scratch
- 0 means restart from unknown run_id (missing -> run_id set to 0)

There may well be a better way to handle missing run_id values in
restart files.

include/bout/solver.hxx

@@ -417,6 +417,12 @@ protected:
   /// Current iteration (output time-step) number
   int iteration{0};
 
+  /// Randomly generated run ID
+  int run_id {1};  // 0 = unknown restart, 1 = not restarted
+  /// The run from which this was restarted.
+  /// Set to zero if no restart
+  int run_restart_from {1};
+
   /// Run the user's RHS function
   int run_rhs(BoutReal t);
   /// Calculate only the convective parts

src/solver/solver.cxx

@@ -468,8 +468,28 @@ int Solver::solve(int NOUT, BoutReal TIMESTEP) {
     throw BoutException(_("Failed to initialise solver-> Aborting\n"));
   }
 
+  // Set the run ID to a random number
+  run_restart_from = run_id; // Restarting from the previous run ID
+
+  if (MYPE == 0) {
+    srand(static_cast<unsigned int>(time(nullptr)));
+    do {
+      run_id = rand(); // Different each time the simulation is run
+    } while((run_id == 0) || (run_id == 1)); // Ensure that run_id != 0 or 1
+  }
+  MPI_Bcast(&run_id, 1, MPI_INT, 0, BoutComm::get()); // All ranks have same run_id
+
+  // Put the run ID into the options tree
+  // Forcing in case the value has been previously set
+  Options::root()["run"]["run_id"].force(run_id, "Solver");
+  Options::root()["run"]["run_restart_from"].force(run_restart_from, "Solver");
+
   /// Run the solver
   output_info.write(_("Running simulation\n\n"));
+  output_info.write("Run ID: {:d}\n", run_id);
+  if (run_restart_from != 0) {
+    output_info.write("Restarting from ID: {:d}\n", run_restart_from);
+  }
 
   time_t start_time = time(nullptr);
   output_progress.write(_("\nRun started at  : {:s}\n"), toString(start_time));
@@ -558,6 +578,10 @@ void Solver::outputVars(Datafile &outputfile, bool save_repeat) {
   outputfile.addOnce(simtime,  "tt");
   outputfile.addOnce(iteration, "hist_hi");
 
+  // Add run information
+  outputfile.addOnce(run_id, "run_id");
+  outputfile.addOnce(run_restart_from, "run_restart_from");
+
   // Add 2D and 3D evolving fields to output file
   for(const auto& f : f2d) {
     // Add to dump file (appending)