Allow for a list of alleles to be used as input for epitope predirction

Jasreet had requested that the epitope prediction pipeline can be run on a list of alleles.
Yeah, I think it would. The downside is that I wouldn't be able to utilize the loop in the _attach_run_netmhc_command subroutine because the property in the RunNetmhc module is called 'allele' (singular). Instead I would need to add:
`$workflow->connect_input(input_property => 'alleles', destination => $run_netmhc_command, destination_property => 'allele');`

Which one do you prefer, @acoffman?