Move textual functions in data_model_utils.tabular to data_model_utils.textual

[HiromuHota]

Description of the problems or issues

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

See #503.

Does your pull request fix any issue.

Fix #503.

Description of the proposed changes

Copy textual functions in data_model_utils.tabular to data_model_utils.textual, and deprecated them in data_model_utils.tabular.

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 #505 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
+ Coverage   85.80%   85.83%   +0.03%     
==========================================
  Files          88       88              
  Lines        4571     4583      +12     
  Branches      855      855              
==========================================
+ Hits         3922     3934      +12     
  Misses        464      464              
  Partials      185      185              

Flag	Coverage Δ
#unittests	85.83% <100.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
src/fonduer/utils/data_model_utils/tabular.py	100.00% <100.00%> (ø)
src/fonduer/utils/data_model_utils/textual.py	72.09% <100.00%> (+14.95%)	⬆️

[lukehsiao]

This reorg makes a lot of sense. Thanks for doing this, and for the nice deprecation warnings!