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_main.py

@@ -172,25 +172,21 @@ class TestMain(TestCase):
         assert len(configs) > 0
 
     def test_until(self):
-        config = {
-            "name": "until_sim",
-            "model_params": {
-                "network_params": {},
-                "agents": {
-                    "fixed": [
-                        {
-                            "agent_class": agents.BaseAgent,
-                        }
-                    ]
-                },
-            },
-            "max_time": 2,
-            "num_trials": 50,
-        }
-        s = simulation.from_config(config)
+        n_runs = 0
+
+        class CheckRun(agents.BaseAgent):
+            def step(self):
+                nonlocal n_runs
+                n_runs += 1
+                return super().step()
+
+        n_trials = 50
+        max_time = 2
+        s = simulation.Simulation(model_params={'agents': [{'agent_class': CheckRun}]},
+                                  num_trials=n_trials, max_time=max_time)
         runs = list(s.run_simulation(dry_run=True))
         over = list(x.now for x in runs if x.now > 2)
-        assert len(runs) == config["num_trials"]
+        assert len(runs) == n_trials
         assert len(over) == 0
 
     def test_fsm(self):