Make file for load scenarios an input

Now the load development scenario (presently 'scenario_LECs_v2.csv') is hard coded in the function get_data_filenames() in the load_mapping.py module. It would be good if it was instead made an input argument together with the other input file names, settings and input arguments for a case, either in some runscript or in some configuration/TOML file. This would make the code more flexible for running multiple scenarios, and it would make it more transparent (and thus reproducible) what inputs are used for a case. (Although ivespe chose to assign this issue to rubir, ivespe is of course available to help:-)