Correct SpectralFlux

[SiggiGue]

Something like this (multichannels not supported in this example)

class SpectralFlux(Feature):
    axis = Parameter(0)
    fmin = Parameter()
    fmax = Parameter()
    
    def requires(self):
        yield AbsRfft

    def on_start(self, source, featureset, sink):
        nfft = featureset['AbsRfft'].nfft
        freqs = featureset['AbsRfft'].frequencies
        fmin, fmax = self.fmin, self.fmax
        if fmin and fmax:
            self._index = np.logical_and(freqs >=fmin, freqs < fmax)
            nfft = np.sum(self._index)
        else: 
            self._index = self._lastspec > 0
        
        if source.channels > 1:
            self._lastspec = np.ones((nfft, source.channels))
        else:
            self._lastspec = np.ones(nfft)

        if not any(self._index):
            raise ValueError('Could not find frequency bins between {} and {} Hz'.format(fmin, fmax))
                    
    def process(self, data, featuredata):
        curspec = featuredata['AbsRfft'][self._index]
        ds = curspec - self._lastspec
        specflux = np.sum((ds + np.abs(ds)) * 0.5)
        self._lastspec = curspec
        return specflux
    
    @classmethod
    def gen_by_edge_frequencies(cls, edgefreqs, prefixname='SpectralFlux'):
        for le, ue in zip(edgefreqs[:-1], edgefreqs[1:]):
            yield cls(name='{}{}to{}Hz'.format(cls.__name__, le, ue), fmin=le, fmax=ue)