Add easy way to create multi-stage OPF and security-constrained OPF

[thomaswolgast]

Both can be manually implemented, but quite complicated. Better would be a standard way to transform an OPF into these more advanced concepts.

Options:

• Gymnasium wrapper: Non-obvious to implement, but probably the most modular way.
• Subclass that inherits from the base class and overwrites step (for multi-stage) or calculate_violations (for security), respectively. Drawback: Too object-oriented. Especially multi-inheritance could become annoying if both need to be combined
• Extend base class: Both are standard OPF variants. Maybe the best way is to implement them directly in the base class. Drawback: even bigger base class, which is not the goal, see Make base class more modular #25.

Important to consider: While the security-constrained OPF is quite clear, there are some open design decisions in the multi-stage OPF. For example, should the agent be able to observe all steps from the beginning, including future steps, or only the current step? Ideally, the implemented option is customizable.

[thomaswolgast]

Added a class for the security-constrained OPF. Still TODO: Add docs and tests.