pdf2txt: clean up construction of LAParams from arguments

[0xabu]

This PR cleans up the way pdf2txt constructs the LAParams object it passes to extract_text_to_fp.

Specifically:

1. Rather than hardcoding the default values of the parameters in two places, have pdf2txt fetch them from the object's existing defaults (all were identical, presumably intentionally so).
2. Rather than passing the parameters as arguments to an intermediate function, construct LAParams closer to the argument parser.
3. Call the LAParams constructor rather than setting attributes after construction, so that LAParams._validate runs to validate the chosen parameters.

As a deliberate side-effect of point 2 above, this fixes the following crash:

$ pdf2txt.py --boxes-flow=disabled test.pdf
Traceback (most recent call last):
  File "tools/pdf2txt.py", line 204, in <module>
    sys.exit(main())
  File "tools/pdf2txt.py", line 198, in main
    outfp = extract_text(**vars(A))
  File "tools/pdf2txt.py", line 66, in extract_text
    pdfminer.high_level.extract_text_to_fp(fp, **locals())
  File "pdfminer/high_level.py", line 85, in extract_text_to_fp
    interpreter.process_page(page)
  File "pdfminer/pdfinterp.py", line 896, in process_page
    self.device.end_page(page)
  File "pdfminer/converter.py", line 51, in end_page
    self.cur_item.analyze(self.laparams)
  File "pdfminer/layout.py", line 822, in analyze
    group.analyze(laparams)
  File "pdfminer/layout.py", line 575, in analyze
    LTTextGroup.analyze(self, laparams)
  File "pdfminer/layout.py", line 362, in analyze
    obj.analyze(laparams)
  File "pdfminer/layout.py", line 575, in analyze
    LTTextGroup.analyze(self, laparams)
  File "pdfminer/layout.py", line 362, in analyze
    obj.analyze(laparams)
  File "pdfminer/layout.py", line 575, in analyze
    LTTextGroup.analyze(self, laparams)
  File "pdfminer/layout.py", line 362, in analyze
    obj.analyze(laparams)
  File "pdfminer/layout.py", line 577, in analyze
    self._objs.sort(
  File "pdfminer/layout.py", line 578, in <lambda>
    key=lambda obj: (1 - laparams.boxes_flow) * obj.x0
TypeError: unsupported operand type(s) for -: 'int' and 'str'

Thus, this supersedes PR #657 (which was a minimal fix for just that crash).

Related: Issue #477, PR #479

Fix #540

How Has This Been Tested?

• I ran pdf2txt --boxes_flow=disabled, and verified that (a) there was no crash, and (b) the alternate layout algorithm was used in LTLayoutContainer.analyze.
• I verified that changing layout parameters alters the output.
• I verified that passing invalid layout parameters now raises an error:

$ tools/pdf2txt.py --boxes-flow 42 samples/simple1.pdf
Traceback (most recent call last):
 File "tools/pdf2txt.py", line 217, in <module>
   sys.exit(main())
 File "tools/pdf2txt.py", line 199, in main
   A = parse_args(args)
 File "tools/pdf2txt.py", line 189, in parse_args
   parsed_args.laparams = LAParams(**kwargs)
 File "pdfminer/layout.py", line 89, in __init__
   self._validate()
 File "pdfminer/layout.py", line 100, in _validate
   raise ValueError(boxes_flow_err_msg)
ValueError: LAParam boxes_flow should be None, or a number between -1 and +1

Checklist

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

[pietermarsman]

Thanks so much! Again a very nice improvement!

Will do some small changes myself and then merge it.

[pietermarsman]

I did some more changes:

• Not using locals() or getattr() anymore when creating LAParams.
• Moved all post-processing of the parsed args to the parse_args() method.
• Also used the default values from LAParams for --detect-vertical and --all-texts. Presumably you omited those by accident?
• Add cli argument for --line-overlap as well.

[0xabu]

LGTM, thanks.