Add errors for Only H1 headings

[94rain]

Describe your changes

• Add errors for Only H1 headings:
  • Logic:
    • No H2 headers:
      • Only H1 headers: Only H1 headings are found. At this time, only H2 heading are supported.
      • No H1 and H2 headers: Expected notebook to have at least one markdown H2 heading.
    • Only one H2 header: Warning: refactoring successful but only one H2 heading detected...
    • Otherwise: run smoothly
• Add a new test case for _get_h2_header (###->None), and a new method _get_h1_header along with tests
• Update test test_find_breaks_error_if_no_h2_headers to test_find_breaks_error_if_no_h2_but_h1_headers and test_find_breaks_error_if_no_h1_and_h2_headers

Issue ticket number and link

Closes #5

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.
  • Update: Fixed the issue of matching H3 or smaller headers as H2. Add error messages when there are only H1 headers.

[edublancas]

looks good, only a small change

[94rain]

looks good, only a small change

Done. Please take another look.

[idomic]

lgtm, nice job!