added parallel processing for prepare_strainge_db.py

[nick-youngblut]

Used for testing

$SCRIPT_PATH=StrainGE/bin/
rm -rf ref_genomes && \
  ncbi-genome-download -p 8 -l complete -t 817 -H -F fasta,assembly-report -o ref_genomes bacteria && \
  rm -rf strainge_db && \
  mkdir strainge_db && \
  python3 $SCRIPT_PATH/prepare_strainge_db.py ref_genomes/human_readable -t 8 -s -o strainge_db > strainge_db/references_meta.tsv