Fetch databases separate from build igblast reference

[ggabernet]

As proposed by @ssnn-airr, it would be nice to separate the process in fetch_databases between pulling the references from IMGT and building the igblast references, to also allow users to provide their own fasta databases but still build them with igblast.

Currently, the fetch_databases.sh script is executed in the Fetch_databases process which calls the underlying scripts:

• fetch_imgt.sh pulls the references directly from IMGT for 4 species and stores them in fasta format.
• fetch_igblastdb.sh gets some internal data needed for igblast
• imgt2igblast.sh uses makeblastdb to create the necessary Igblast references.

That could be separated into two processes, one for pulling the fasta references, and one for building the necessary references for igblast.

So if the reference data is available in IMGT, one could adapt the first script to also pull it and then build the igblast needed references. Or if custom reference data is available in fasta format, it should also be possible to provide it directly to imgt2igblast to build the references.