Compute the central time for each passes.
        
        .. note:: this must be called AFTER having called :meth:`altimetry.data.alti_data.reorder` as it looks for the CYCLE and RECORD dimensions.
        .. note:: The methodology to compute the central time is to interpolate the time along the track at missing points, and then reading the value at point N/2.

def pass_time(self):
        '''
        Compute the central time for each passes.
        
        .. note:: this must be called AFTER having called :meth:`altimetry.data.alti_data.reorder` as it looks for the CYCLE and RECORD dimensions.
        .. note:: The methodology to compute the central time is to interpolate the time along the track at missing points, and then reading the value at point N/2.
        '''
        date=self.date
        nt=self._dimensions['cycle']
        N=self._dimensions['record']
        for t in np.arange(nt):
            poly=np.polyfit(np.arange(N)[~date.mask[t,:]], date[t,:][~date.mask[t,:]], 1)
            date[t,:][date.mask[t,:]]=poly[0]*np.arange(N)[date.mask[t,:]] + poly[1]
        date.mask=False
        return date[:,N/2]