Rename "VisualLinker" to "PdfVisualParser" to welcome "HocrVisualParser"

[HiromuHota]

Description of the problems or issues

Is your pull request related to a problem? Please describe.

As #509 now became a big PR, I'll break it down into smaller PRs.
To support hOCR (#476), this PR renames VisualLinker to PdfVisualParser to welcome HocrVisualParser, which will be submitted in a separate PR.

Does your pull request fix any issue.

N/A.

Description of the proposed changes

• Renamed VisualLinker to PdfVisualParser, which takes pdf_path as an argument
  • pdf_path now should be a path to a directory containing PDF files.
  • The PDF file's name should be the same as document_name + ".pdf" (or ".PDF")
• Changed Parser's signature:
  • Renamed vizlink to visual_parser.
  • Removed pdf_path. Now this is required only by PdfVisualParser.
  • Removed visual. Use visual_parser if visual information is to be parsed.

Test plan

A clear and concise description of how you test the new changes.

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the CHANGELOG.rst accordingly.

[codecov-commenter]

Codecov Report

Merging #518 into master will increase coverage by 0.08%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
+ Coverage   85.72%   85.81%   +0.08%     
==========================================
  Files          88       90       +2     
  Lines        4583     4582       -1     
  Branches      856      852       -4     
==========================================
+ Hits         3929     3932       +3     
+ Misses        468      467       -1     
+ Partials      186      183       -3     

Flag	Coverage Δ
#unittests	85.81% <94.44%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/fonduer/parser/visual_parser/visual_parser.py	80.00% <80.00%> (ø)
src/fonduer/parser/parser.py	93.15% <100.00%> (+1.07%)	⬆️
src/fonduer/parser/visual_parser/__init__.py	100.00% <100.00%> (ø)
.../fonduer/parser/visual_parser/pdf_visual_parser.py	84.76% <100.00%> (ø)

[senwu]

Left two comments. Otherwise looks good to me.

[HiromuHota]

@senwu thanks for your review and comments. Please review again and merge it when it's ready.