iaf_psc_exp_multisynapse is a direct extension of iaf_psc_exp.
On the postsynapic side there can be arbitrarily many synaptic
time constants (iaf_psc_exp has exactly two: tau_syn_ex and tau_syn_in).
This can be reached by specifying separate receptor ports each for
a different time constant. The port number has to match the respective
"receptor_type" in the connectors.