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)
I guess I still don't have confidence about the (1) the range of weird shit we'll find in VCF and MAF files and (2) the possible outputs we'll see from NetMHCcons. So, when problems come up it's convenient to have nearby dumps of internal state. Maybe I should do another PR to switch to an explicit "--verbose" flag?