display warning when only one H2 head detected.

[Wxl19980214]

Describe your changes

Added a warning message if only one H2 head detected

Issue ticket number and link

Closes #47

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I have added thorough tests (when necessary).
• [x] I have added the right documentation (when needed). Product update? If yes, write one line about this update.

[edublancas]

can you add a test case? you can create a sample notebook with one H2 heading and check that the message is displayed: https://docs.pytest.org/en/6.2.x/capture.html

[idomic]

@Wxl19980214 I see some linting ci issues, please fix.

[Wxl19980214]

test added and lint fixed.

[idomic]

Nice job!