Calculate Averaged CONGU and CONGL

[rouille]

• Implement calculate_averaged_congestion in the postreise.extract.extract_data module that calculates the time averaged CONGU and CONGL for each branch.
• Write tests.
• Use the above function to generate an output pickle file enclosing the statistics, e.g., i_AVERAGED_CONG.pkl where: i is the scenario id. The data frame will have branch_id as index and CONGL and CONGU as columns.
• Implement the get_averaged_cong method in the Analyze object that download/load the file.
• Add the download/load functionality to the OutputData object.