Use the centroid to check if textlines are within a bbox

[HiromuHota]

Description of the problems or issues

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

See #96

Does your pull request fix any issue.

Fix #96

Description of the proposed changes

Use the centroid to check if textlines are within a bbox instead of its whole bbox. This change makes the isContained check loose, which tolerates a fluctuating bbox returned from Tabula.

Test plan

Check if cell values are not missed and extracted.

Checklist

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