Convert multiple BLAST XML files to single tabular file (by JJ + PJC)

Based on changes by Jim Johnson for the Galaxy-P instance at
University of Minnesota, revised and merged by Peter Cock.
Refactor to avoid global blast_program and outfile

This is mostly an indentation change, git diff -w at the command line quite short.
Select columns in blastxml_to_tabular.py (by JJ + PJC)

Based on changes by Jim Johnson for the Galaxy-P instance at
University of Minnesota, revised and merged by Peter Cock.
Column selection in blastxml_to_tabular.xml (by JJ + PJC)

Based on changes by Jim Johnson for the Galaxy-P instance at
University of Minnesota, revised and merged by Peter Cock.