Make Popen process its own group leader

Otherwise, when running in Jupyter on Mac, the ipykernel will have the
same group ID, and will be killed when the Popen process's group ID is
killed. Fixes Parsl/parsl#212.

libsubmit/channels/local/local.py

@@ -123,7 +123,8 @@ def execute_no_wait(self, cmd, walltime, envs={}):
                 stderr=subprocess.PIPE,
                 cwd=self.userhome,
                 env=current_env,
-                shell=True
+                shell=True,
+                preexec_fn=os.setpgrp
             )
             pid = proc.pid