Include month resolution to hurdle rates

[PascalLesage]

Includes:

• data and result loading and writing for monthly resolution Tx hurdle rates
• monthly resolution in hurdle-related sets and constraints
• functionality to set monthly values to a single period value if month is set to 0
• two hurdle rate csv files for testing
• one test
  Not working:
• hurdle costs aggregation
  Not tested:
• test_data_loaded_correctly written but not run.
• no other test run

[anamileva]

@PascalLesage, thanks for this, it looks good and should be usable on your end. The tests are failing because a hurdle rate subscenario with no months is missing (so we get a foreign key error for the new test_hurdle_rates_no_months scenario when creating the test database). I fixed this locally by adding a new CSV file db/csvs_test_examples/transmission/transmission_hurdle_rates/2_tx_hurdle_rates_no_months.csv like this:

transmission_line,period,month,hurdle_rate_positive_direction_per_mwh,hurdle_rate_negative_direction_per_mwh
Tx_1,2020,0,5,5
Tx_1,2025,0,5,5
Tx_new,2020,0,5,5
Tx_new,2025,0,5,5

You can add this new example to tests/test_examples.py also. Finally, the hurdle costs module test (tests/transmission/operations/test_hurdle_costs.py) should be updated to take into account that we now also have the month index. It should be good to merge after that.