ISI coefficient of variation

[anonymous]

in signals.py: - when defining the CV of a SpikeTrain with less than 2 spikes, should issue a warning but return a NaN (instead of stopping the script by raising an error)

- when defining the CV of a SpikeList, should use the code from the SpikeTrain and return a list of CVs, like:

    def cv_isi(self):
        """
        Return the list of all the cv coefficients for all the SpikeTrains objects
        within the SpikeList.
        
        See also:
            cv_isi_hist, cv_local, cv_kl
        """
        cvs_isi = numpy.empty(self.N)
        for id in self.id_list():
            cvs_isi[id] = self.spiketrains[id].cv_isi()
        return cvs_isi

every empty SpikeTrain, will issue a warning

[pierre]

Should be fixed. I add a flag to the cv_isi() method that will allow or not the NaN values. They are allowed by default. Regarding the error messages, I put them into the logging file, otherwise it's really annoying in a big SpikeList where you would have a lot of non spiking enough cells