Join worker processes in simulate_many()

Although the worker processes are in daemon mode, and although we know
these daemon processes have posted their results to the result queue and
are thus done simulating, there still may be cleanup code that the
processes are running.

Specifically, this has been seen to effect coverage where coverage data
files can be partially written.

desmod/simulation.py

@@ -287,13 +287,15 @@ def simulate_many(configs, top_type, env_type=SimEnvironment, jobs=None):
     if jobs is not None:
         num_workers = min(num_workers, jobs)
 
+    workers = []
     for i in range(num_workers):
         worker = Process(name='sim-worker-{}'.format(i),
                          target=_simulate_worker,
                          args=(top_type, env_type, False, progress_queue,
                                config_queue, result_queue))
         worker.daemon = True    # Workers die if main process dies.
         worker.start()
+        workers.append(worker)
         config_queue.put(None)  # A stop sentinel for each worker.
 
     if progress_enable:
@@ -312,6 +314,9 @@ def simulate_many(configs, top_type, env_type=SimEnvironment, jobs=None):
         # progress_thread is still using it.
         progress_thread.join(1)
 
+    for worker in workers:
+        worker.join(5)
+
     return sorted(results, key=lambda r: r['config']['meta.sim.index'])