Test & improve perfomance in .tools.add_year

[khaeru]

Most (137 of 187) untested lines in message_ix are in .tools.add_year: https://codecov.io/gh/iiasa/message_ix/tree/master/message_ix/tools/add_year

This PR adds tests for this code, but also reimplements some internal methods for better performance.

Partially addresses #433.

How to review

To be added.

PR checklist

• Complete code
  • Remove usage of pandas.DataFrame.append(), which was removed in pandas 2.0.0.
• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Attention: Patch coverage is 92.07921% with 8 lines in your changes missing coverage. Please review.

Project coverage is 95.0%. Comparing base (a824be9) to head (7a3d2ba).
Report is 407 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix/tools/add_year/__init__.py	88.8%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #494     +/-   ##
=======================================
+ Coverage   94.4%   95.0%   +0.6%     
=======================================
  Files         43      43             
  Lines       3432    3482     +50     
=======================================
+ Hits        3241    3310     +69     
+ Misses       191     172     -19     

Files with missing lines	Coverage Δ
message_ix/tests/tools/test_add_year.py	96.9% <100.0%> (+1.1%)	⬆️
message_ix/tools/add_year/__init__.py	72.8% <88.8%> (+6.6%)	⬆️

[glatterf42]

Filtering out commits that were included in #703 already.