add comments to explain what the function is doing

[12rambau]

To avoid maintenance issues in the future please add as many comments as you can in your functions to explain what you're doing:

• [x] https://github.com/12rambau/sdg_indicators_module/blob/c8b7e2ea46d1b25e6927bf1c3fa61bb5dfccdfdd/component/scripts/run_15_3_1.py#L182
• [x] https://github.com/12rambau/sdg_indicators_module/blob/c8b7e2ea46d1b25e6927bf1c3fa61bb5dfccdfdd/component/scripts/run_15_3_1.py#L212