Markup <table/> and <td/>

[HiromuHota]

Description of the problems or issues

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

Currently, <table/> and <td/> are not marked up with hOCR attributes.

Does your pull request fix any issue.

N/A

Description of the proposed changes

This PR adds

• title="bbox X X X X" to both of them.
• class_="ocr_table" to <table/>

Test plan

• Use hocr-checks to check the conformity to hOCR (this check is actually very basic)
• Check if table can be found by class "ocr_table"
• Check if they have bbox

Checklist

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

[codecov-io]

Codecov Report

Merging #84 into master will increase coverage by 0.23%. The diff coverage is 79.59%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   65.62%   65.86%   +0.23%     
==========================================
  Files          21       21              
  Lines        2508     2525      +17     
==========================================
+ Hits         1646     1663      +17     
  Misses        862      862              

Flag	Coverage Δ
#unittests	65.86% <79.59%> (+0.23%)	:arrow_up:

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

Impacted Files	Coverage Δ
pdftotree/ml/TableExtractML.py	0.00% <0.00%> (ø)
pdftotree/utils/pdf/render.py	0.00% <0.00%> (ø)
pdftotree/utils/pdf/pdf_parsers.py	92.38% <14.28%> (-0.04%)	:arrow_down:
pdftotree/utils/pdf/grid.py	20.90% <33.33%> (+0.90%)	:arrow_up:
tests/test_basic.py	95.52% <88.46%> (+0.52%)	:arrow_up:
pdftotree/TreeExtract.py	88.85% <95.65%> (+0.23%)	:arrow_up:
pdftotree/core.py	100.00% <100.00%> (ø)
pdftotree/ml/features.py	63.82% <100.00%> (ø)

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