Split into submodules

[daemontus]

The single-module architecture is starting to look a bit crowded. Some suggestions for (future) modules: bdd, model, attractors, fixed_points, trap_spaces, control, ...

[daemontus]

Currently, this is postponed. The reason is that it is very hard to make native submodules work with Python documentation engines correctly.

Plan for v2.0 would be:

• Include Python bindings directly into native libraries (data transfer between native libraries would have to be through copying).
• Make AEON.py a pure Python library that calls into the native libraries.

Then, we could use all the normal documentation tools and type annotations without major compatibility problems..