Compress FASTA files with contigs and scaffolds using bgzip instead of gzip, to enable file indexing

[amizeranschi]

Description of feature

Certain downstream analysis tools expect the FASTA files with contigs and scaffolds to be compressed with bgzip instead of gzip, for file indexing purposes. To save time with decompressing and recompressing, it would be helpful if the MEGAHIT and SPAdes modules would directly compress their outputs with bgzip instead of gzip.