Add an LC filter model, refactoring

[mhinkkan]

This pull request contains:

• A model for an LC output filter is added. Due to this addition, the converter output voltage is now marked with u_c (or u_cs in stator coordinates) in all system models (even if no LC filter were included). This change in the notation simplifies inheritance.
• A simple example of an LC-filtered induction motor drive is provided, controlled using open-loop V/Hz control.
• A helper function wrap is used in control algorithms to simplify the limitation of angles between -pi and pi.