Feat: Add logic and test code for add_edge, add_directed_edges, add_undirected_edges methods of PDAG

[daehyun99]

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Issue number(s) that this pull request fixes

• Fixes [ENH] Implement the add_directed_edges and add_undirected_edges methods of the PDAG class. #2311

List of changes to the codebase in this pull request

• Test: Add tests code for add_edge, add_directed_edges, add_undirected_edges methods of PDAG
• Feat: Add logic code for add_edge, add_directed_edges, add_undirected_edges methods of PDAG

[daehyun99]

This PR depends on PR Feat: Add logic and test code for add_node, add_nodes_from method of PDAG #2310

[fkiraly]

same comment as in #2310 (review)

[daehyun99]

The methods related to edge will be discussed in issue #2330.
I am closing this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.02%. Comparing base (2bba5dc) to head (1872c71).
⚠️ Report is 87 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2312      +/-   ##
==========================================
+ Coverage   94.59%   95.02%   +0.43%     
==========================================
  Files         149      158       +9     
  Lines       20155    22245    +2090     
==========================================
+ Hits        19065    21138    +2073     
- Misses       1090     1107      +17     

Files with missing lines	Coverage Δ
pgmpy/base/ADMG.py	89.95% <ø> (ø)
pgmpy/base/DAG.py	93.15% <100.00%> (+0.54%)	⬆️
pgmpy/base/__init__.py	100.00% <100.00%> (ø)
pgmpy/tests/test_base/test_DAG.py	100.00% <100.00%> (ø)

... and 36 files with indirect coverage changes