Start processes if not already started

def _init_proc(self):
        """Start processes if not already started"""
        if not self.proc:
            self.proc = [
                mp.Process(target=self._proc_loop, args=(i, self.alive, self.queue, self.fn))
                for i in range(self.num_proc)
            ]
            self.alive.value = True
            for p in self.proc:
                p.start()