Improve flexibility of emit_spike()

[clinssen]

emit_spike() sends out an event, that can be optionally tagged with data fields like weight and delay, for instance: emit_spike(my_weight, 1 ms).

We should create a more flexible syntax and semantics to allow arbitrary data fields, with given types, to be attached to emitted events, for instance: emit_spike(w=my_weight, d=1 ms).

This could be "read out" on the receiving side as follows (for instance):

onReceive(spikes_in):
  V_m += spikes_in.w

[clinssen]

Duplicate of #946.