Use a different temp directory for each NetMHCcons instance, moved

mutate from epitopes into immuno, parse stdout of NetMHCcons instead of
xls files (to avoid clobbering hardcoded output file path)
Gotcha. In general, casting any random input in a string concerns me in terms of the fail-fast approach. Can we say something like: if int, cast to string, with an associated comment about why we're doing that? (Or, at the least, a comment.)

Also, can we have a unit test for this weird int issue?