Improve internal code structure of controller classes (#47)

* Improve internal code structure of controllers

- For both `LTIDataDrivenMPCController` and
  `NonlinearDataDrivenMPCController` classes:
  - Rename internal attributes to include a leading underscore for
    clarity.
  - Move public methods at the top and internal ones at the bottom.
- Update tests to align with the renaming of internal attributes and
  methods.

These changes ensure code follows Python development best practices
and improve clarity.

* Use PEP 526 attribute annotations for controller classes

This improves attribute type annotations and aligns the code with
modern Python development practices.

Author: pavelacamposp