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).

tests/test_config.yml

@@ -0,0 +1,5 @@
+---
+source_file: "../examples/torvalds_sim.py"
+model: "TorvaldsEnv"
+max_steps: 10
+interval: 2