Ensure lons and lats are:
         - 1D numpy arrays
         - equal size
         - within the appropriate range in radians

def _check_integrity(self, lons, lats):
        """
        Ensure lons and lats are:
         - 1D numpy arrays
         - equal size
         - within the appropriate range in radians
        """

        lons = np.array(lons).ravel()
        lats = np.array(lats).ravel()

        if len(lons.shape) != 1 or len(lats.shape) != 1:
            raise ValueError('lons and lats must be 1D')
        if lats.size != lons.size:
            raise ValueError('lons and lats must have same length')
        if (np.abs(lons)).max() > 2.*np.pi:
            raise ValueError("lons must be in radians (-2*pi <= lon <= 2*pi)")
        if (np.abs(lats)).max() > 0.5*np.pi:
            raise ValueError("lats must be in radians (-pi/2 <= lat <= pi/2)")
        return lons, lats