Modified ANIb method better to match JSpecies

The previously implemented BLASTN+ method did not correspond very well
with the legacy BLASTN method used in JSpecies (http://www.imedea.uib.es/jspecies/).
Two main changes were made.

1) The BLASTN command now uses -task blastn, to modify wordlength and
   other parameters to better match legacy BLASTN. This also required
   changes to gapopen and gapextend parameters (i.e. their removal)
2) As the max_target_seqs and num_alignments parameters cannot restrict
   tabular output to a single match, this is now managed in the script.

ANIb output is now a much better match for JSpecies, but some differences
remain. These appear to have been minor in testing.