New implementation of the SpikeGenerator with the modulo argument (except in Weave, don't know how to implement it...) The implementation
should be more robust to corner cases. However, special treatments need to be performed during the last time step, just before the period is over,
to avoid missed spikes
Use Marcel's patch to allow the use of infinity as a default period, ans also make use of
fmod in the weave implementation, such that this is the same as the C++ one