Fix ordering of grouping textboxes. The first grouping of textboxes should be skipped if there are intermediate textboxes.

[pietermarsman]

Description

#315 introduces a bug by replacing 0 by True and 1 by False in the ordening of textbox distances. Consequently, all the text boxes that are checked once (for intermediate textboxes) have priority over those that are never checked. Therefore it merges the same text box over and over again.

@mikkkee, can you have a look at this?

Fixes #334

How Has This Been Tested?

• I've use the pdf in Version pdfminer.six 20191107 incorrectly orders some text #334 to check the output
• I've added a debug log statement to see the order in which text boxes are grouped.
• The doctests in the documentation improved
• Check speedup improvement from Layout speedup rebased #315

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have updated the README.md and other documentation, or I am sure that this is not necessary
• I have added a consice human-readable description of the change to CHANGELOG.md
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial version

[mikkkee]

After figuring out what has happened, I think this PR is fine. I can confirm the speed up is still there.

The current develop branch failed using my test script in #315 with RecursionError: maximum recursion depth exceeded. Shall we add my 01.pdf test as an unit test?