021 ncbi fetch

blast: support for remote (ncbi) blast

ncbi_fetch (new): Given list of seqids (gi or accession numbers) fetch their corresponding sequences from ncbi
It would be better to have two separate options for specifying a database:
-d/--database (a path to a database or None) and -D/--remote-database (with choices nt, nr, etc.) This way --database does not need to be overloaded to take paths vs remote db names, and you can enforce remote database names as choices within argparse. --database would no longer be required, but one of --database or --remote-database would be.