use dill to store the spawn function as bytes

when creating a worker, we pass in a spawn function. Dill is an improved version of pickle so dill can serialize many
more functions than pickle can. Thus we use dill to serialize the spawn function to bytes, which can then be serialized/deserialized by pickle when
spawning subprocesses. Then when the worker needs to run the spawn function, we can use dill to deserialize the bytes into the desired function.

daisy/worker.py

@@ -4,6 +4,7 @@
 import multiprocessing
 import os
 import queue
+import dill
 
 logger = logging.getLogger(__name__)
 
@@ -50,6 +51,14 @@ def __init__(self, spawn_function, context=None, error_queue=None):
 
         self.start()
 
+    @property
+    def spawn_function(self):
+        return dill.loads(self._spawn_function)
+
+    @spawn_function.setter
+    def spawn_function(self, value):
+        self._spawn_function = dill.dumps(value)
+
     def start(self):
         """Start this worker. Note that workers are automatically started when
         created. Use this function to re-start a stopped worker."""