Add services with either write or read but not both

[amascare]

During my PML modeling, I have noticed that in some cases it is convenient to create Transporters which serve only loads or stores.
Currently, I deal with this situation by passing the Transporter a set of two variables (one is a load and the other a store) and then unlinking when necesary, e.g., SimpleTransporter(Set(ld_requests, str_requests)), where ld_requests is Load() and str_requests is Store().
I think that having the possibility of creating Transporters with the capability of serving either one or the other would simplify the process.