def _iter_native_dataset(self, native_filters=None):
raise NotImplementedError<EOL>
To be implemented by subclass.
Must be a generator.
Must yield a callable, *native_quantity_getter*.
This function must iterate over subsets of rows, not columns!

*native_filters* will either be `None` or a `GCRQuery` object
(unless `self.native_filter_string_only` is set to True, in which
case *native_filters* will either be `None` or a tuple of strings).
If *native_filters* is a GCRQuery object, you can use
`native_filters.check_scalar(scalar_dict)` to check if `scalar_dict`
satisfies `native_filters`.

Below are specifications of *native_quantity_getter*
-----------------------------------------
Must take a single argument of a native quantity name
(unless `_obtain_native_data_dict` is edited accordingly)
Should assume the argument is valid.
Must return a numpy 1d array.