Moved all data files from Data/ to Data/US/ to denote the fact that they contain data for the US only. In the future, data for other countries may be added in subdirectories of Data/.

Similarly, Population_and_tower_data/ has been moved to Data/US/Population_and_tower_data/ to denote the fact that these files contain data for the US only.

get_simulation_value() has been updated accordingly to return the updated values for 'data_dir', 'population_data_dir', and 'tower_data_dir'.

run_me_first.m has been updated to first reset the Matlab path and then add only the data paths for the current region code (which is set in get_simulation_value.m).