Let TableExtractorML inherit TreeExtractor to use its updated parse()

[HiromuHota]

Description of the problems or issues

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

See #105

Does your pull request fix any issue.

Fix #105

Description of the proposed changes

TableExtractorML.parse() should have been updated when TreeExtractor.parse() got updated at #79.
As TableExtractorML and TreeExtractor share some functionally identical methods, I let TableExtractorML inherit TreeExtractor to avoid duplicate of codes and to let it use the updated parse() method.

Test plan

Manually run https://github.com/HazyResearch/pdftotree#example-dataset-paleontological-papers

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.md accordingly.

[codecov-io]

Codecov Report

Merging #106 into master will increase coverage by 2.17%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   67.69%   69.86%   +2.17%     
==========================================
  Files          24       24              
  Lines        2600     2519      -81     
==========================================
  Hits         1760     1760              
+ Misses        840      759      -81     

Flag	Coverage Δ
unittests	69.86% <0.00%> (+2.17%)	⬆️

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)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cb7b074...9e88964. Read the comment docs.