Assign outfile to a variable

This is in prevision to output mapped reads to /dev/null
if -dry is true.
Do Gather_File if -dry is false
Code cleanup
Nodes do not write bowtie temp results to disk if -dry

If the dry run option is set, each node will output bowtie's temporary
outputs to /dev/null (instead of <workdir>/pmapoutfile.out.txt), thus
preventing useless disk writing. Additionally, no output will be sent
to the head node for concatenation.
Extend /dev/null usage to other mappers