Add attributes to spike events

[clinssen]

Fixes #946.
Fixes #944.

• The processing of spikes in the FIR filter is now delayed by one resolution timestep. The impact and/or mitigations of this should still be discussed.
• The spike event handle() method now needs to store each received spike in a separate buffer entry, otherwise excitatory and inhibitory spikes (which are distinguished by sign of the weight); if they are not in a separate buffer, an inhibitory spike with weight -4 and excitatory spike of weight 1 would end up as an inhibitory spike of -3). A consequence of this is more buffering is needed, which could result in a performance hit. A code generator option should be introduced, that gives a list of spiking input port names, such that for each port, it allows the regular RingBuffers to be used.