[Enh] Reuse Elephant function for rasterplot_rates

[kohlerca]

Looking into the code for the rasterplot_rates function, the computation of the population histogram uses the matplotlib hist function (lines 465 and 469 of rasterplot.py).

It would be useful to reuse the time_histogram function from Elephant, which provides more options to control the histogram, especially the ability to normalize by either mean or density.