Fix multithreading

Multithreading needs pickling to work.
Pickling/unpickling didn't work in some situations, like when the
environment_agents parameter was left blank.
This was due to two reasons:

1) agents and history didn't have a setstate method, and some of their
attributes cannot be pickled (generators, sqlite connection)
2) the environment was adding generators (agents) to its state.

This fixes the situation by restricting the keys that the environment exports
when it pickles, and by adding the set/getstate methods in agents.

The resulting pickles should contain enough information to inspect
them (history, state values, etc), but very limited.

tests/test_main.py

@@ -2,6 +2,7 @@ from unittest import TestCase
 
 import os
 import yaml
+import pickle
 import networkx as nx
 from functools import partial
 
@@ -248,12 +249,10 @@ class TestMain(TestCase):
         assert name == 'soil.agents.BaseAgent'
         assert ser == agents.BaseAgent
 
-        class CustomAgent(agents.BaseAgent):
-            pass
-
         ser, name = utils.serialize(CustomAgent)
         assert name == 'test_main.CustomAgent'
         assert ser == CustomAgent
+        pickle.dumps(ser)
 
     def test_serialize_builtin_types(self):
 
@@ -269,7 +268,8 @@ class TestMain(TestCase):
         assert ser == 'test_main.CustomAgent'
         ser = agents.serialize_type(agents.BaseAgent)
         assert ser == 'BaseAgent'
-
+        pickle.dumps(ser)
+    
     def test_deserialize_agent_distribution(self):
         agent_distro = [
             {
@@ -284,6 +284,7 @@ class TestMain(TestCase):
         converted = agents.deserialize_distribution(agent_distro)
         assert converted[0]['agent_type'] == agents.CounterModel
         assert converted[1]['agent_type'] == CustomAgent
+        pickle.dumps(converted)
 
     def test_serialize_agent_distribution(self):
         agent_distro = [
@@ -299,6 +300,20 @@ class TestMain(TestCase):
         converted = agents.serialize_distribution(agent_distro)
         assert converted[0]['agent_type'] == 'CounterModel'
         assert converted[1]['agent_type'] == 'test_main.CustomAgent'
+        pickle.dumps(converted)
+
+    def test_pickle_agent_environment(self):
+        env = Environment(name='Test')
+        a = agents.BaseAgent(environment=env, agent_id=25)
+
+        a['key'] = 'test'
+
+        pickled = pickle.dumps(a)
+        recovered = pickle.loads(pickled)
+
+        assert recovered.env.name == 'Test'
+        assert recovered['key'] == 'test'
+        assert recovered['key', 0] == 'test'
 
     def test_history(self):
         '''Test storing in and retrieving from history (sqlite)'''