devops tests for notebook sizes (not to accept large files into the repo)

[slayoo]

something like this would do:

def test_file_size(notebook):
    """ checks if all example Jupyter notebooks have file size less than a certain limit """
    assert os.stat(notebook).st_size * si.byte < 4.5 * si.megabyte

[github-actions[bot]]

Stale issue message

[slayoo]

addressed in #17