Parallel time windows

Update the ML workflow to parallelize time window analyzes: Example of command:

nextflow run main.nf --mode ml  --results results --nb_max 5000 \
          --sample_min_date 2021-09-01 --sample_max_date 2022-09-01 \
          --window_step 1 --window_size 4 \
          --country France \
          --fasta sequences.tar.xz \
          --metadata metadata.tar.xz \
          --fasta_ref ref_sequences.fasta \
          --metadata_ref ref_metadata.tsv \
          --dates_ref ref_dates.tsv