Returns a list of ndarrays.

        Keyword args:
        field_name -- raster field name as str

def arrays(self, field_name=None):
        """Returns a list of ndarrays.

        Keyword args:
        field_name -- raster field name as str
        """
        fieldname = field_name or self.raster_field.name
        arrays = []
        for obj in self:
            arr = getattr(obj, fieldname)
            if isinstance(arr, np.ndarray):
                arrays.append(arr)
            else:
                arrays.append(obj.array())
        return arrays