correlograms are now normalized
added experiments with numba to accelerate the computation of the cross-correlograms. Apparently, with the current algorithm, Numba cannot do better than the Python/Numpy version. This is probably because the bottleneck of the algorithm is in the computation of the histograms, which is already completely optimized in Numpy. So the solution to accelerate the computation of the cross-correlograms probably involves parallelization of this algorithm, or at least using an external thread to avoid blocking the UI. Or we could come up with an optimized algorithm adapted to the computation of all pairwise correlograms within a pool of neurons.