Ensure reproducibility even with intermediate results #73
Description
Several steps can make the results non reproducible if partial files exist:
- the init_stats.csv file is read if it exists, but it may not contain all the DEMs -> missing DEMs should be appended, or the file re-calculated if missing entries
- the ddem file is not recreated if it exists -> it should be re-created if any of the inputs is newer, or if new DEM selection is used