Large set of changes for v0.30

The examples weren't being properly tested in the last commit. When we fixed
that a lot of bugs in the new implementation of environment and agent were
found, which accounts for most of these changes.

The main difference is the mechanism to load simulations from a configuration
file. For that to work, we had to rework our module loading code in
`serialization` and add a `source_file` attribute to configurations (and
simulations, for that matter).

examples/random_delays/random_delays_sim.py

@@ -40,7 +40,7 @@ s = Simulation(
     model=RandomEnv,
     num_trials=1,
     max_time=100,
-    dry_run=True,
+    dump=False,
 )