Support spikes in sbi

[mstimberg]

Currently, the sbi Inferencer always works on traces, but it should also work on spikes (cf. SpikeFitter). Internally, this should already mostly work if the name of the output variable is set to spikes. Note that features will no longer receive an array, but instead a list of arrays, since for each parameter combination/input, there can be a different number of spikes.