FEAT: Add validation to TrianglePandas.rename. Also add tests to ensure cascading of rename to triangle.columns_label.

[genedan]

Closes #654. Partially addresses #655 regarding the columns_label issue.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.10%. Comparing base (0341334) to head (08b7a95).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
chainladder/core/pandas.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
+ Coverage   85.05%   85.10%   +0.05%     
==========================================
  Files          85       85              
  Lines        4865     4868       +3     
  Branches      619      619              
==========================================
+ Hits         4138     4143       +5     
+ Misses        518      517       -1     
+ Partials      209      208       -1     

Flag	Coverage Δ
unittests	85.10% <88.88%> (+0.05%)	⬆️

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[henrydingliu]

can you please add a test for the new exception?

[genedan]

k, added a few tests cases (incorrect # columns, misspelled strings, incorrect integer supplied).