Trust-region algorithm

[stroitzsch]

Implement trust-region algorithm for operation problem solution.

• [x] Check existing code in examples/development/run_electric_grid_optimal_power_flow_trust_region.py
• [x] Revise implementation to work with latest linear model changes (change values -> absolute values)
• [x] Make it work for multiple timesteps
• [x] Blocking: Local linear electric grid model #5 @stroitzsch
• [x] Blocking: DLMPs with a "different" linear_electric_grid_model for each timestep @stroitzsch
• [x] Make it work with local linear model
• [ ] Move implementation to problems module (exact location TBD)

[tomschelo]

@stroitzsch There is another ToDo for obtaining the DLMPs from the linear_electric_grid_model. This must be done with a "different" linear_electric_grid_model for each timestep as the matrices vary over time.

[stroitzsch]

Thanks for highlighting, I added it above!