Refactored time

Treating time and conditions as the same entity was getting confusing, and it
added a lot of unnecessary abstraction in a critical part (the scheduler).

The scheduling queue now has the time as a floating number (faster), the agent
id (for ties) and the condition, as well as the agent. The first three
elements (time, id, condition) can be considered as the "key" for the event.

To allow for agent execution to be "randomized" within every step, a new
parameter has been added to the scheduler, which makes it add a random number to
the key in order to change the ordering.

`EventedAgent.received` now checks the messages before returning control to the
user by default.

tests/test_time.py

@@ -30,8 +30,9 @@ class TestMain(TestCase):
 
         times_started = []
         times_awakened = []
+        times_asleep = []
         times = []
-        done = 0
+        done = []
 
         class CondAgent(agents.BaseAgent):
 
@@ -39,36 +40,38 @@ class TestMain(TestCase):
                 nonlocal done
                 times_started.append(self.now)
                 while True:
-                    yield time.Cond(lambda agent: agent.model.schedule.time >= 10)
+                    times_asleep.append(self.now)
+                    yield time.Cond(lambda agent: agent.now >= 10,
+                                    delta=2)
                     times_awakened.append(self.now)
                     if self.now >= 10:
                         break
-                done += 1
+                done.append(self.now)
 
         env = environment.Environment(agents=[{'agent_class': CondAgent}])
 
 
         while env.schedule.time < 11:
-            env.step()
             times.append(env.now)
+            env.step()
+
         assert env.schedule.time == 11
         assert times_started == [0]
         assert times_awakened == [10]
-        assert done == 1
+        assert done == [10]
         # The first time will produce the Cond.
-        # Since there are no other agents, time will not advance, but the number
-        # of steps will.
-        assert env.schedule.steps == 12
-        assert len(times) == 12
+        assert env.schedule.steps == 6
+        assert len(times) == 6
 
-        while env.schedule.time < 12:
-            env.step()
+        while env.schedule.time < 13:
             times.append(env.now)
+            env.step()
 
-        assert env.schedule.time == 12
+        assert times == [0, 2, 4, 6, 8, 10, 11]
+        assert env.schedule.time == 13
         assert times_started == [0, 11]
-        assert times_awakened == [10, 11]
-        assert done == 2
+        assert times_awakened == [10]
+        assert done == [10]
         # Once more to yield the cond, another one to continue
-        assert env.schedule.steps == 14
-        assert len(times) == 14
+        assert env.schedule.steps == 7
+        assert len(times) == 7