Do not assume equally sampled x-data

[jvdd]

The current code assumes equally sampled x-data (bin size is expressed in number of samples - assuming there are no gaps & sampling rate does not change)

To remove this assumption we should use something like np.searchsorted to determine the start- and end-indexes of the bins (this uses the x-data values to determine the bins).

I guess for empty bins, we should also perform some cleanup?