tweaks

---------------

When import a class / function within the same package, a ``relative import``

should be used instead of ``absolute import``.

An example is below (``top_level_interface.py``):

An example is below (``api.py``):

.. code-block:: python

    from .flip_image import flip_image, read_test_image

    from .demo import a_demo_function, ADemoClass

    __all__ = ["flip_image", "read_test_image", "a_demo_function", "ADemoClass"]

.. literalinclude:: ../../csst_proto/top_level_interface.py

   :linenos:

   :language: python

.. note::

    Note the ``.`` means it's a relative import.