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

❌ Patch coverage is 6.12245% with 92 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.5%. Comparing base (d7452ee) to head (91cdca4).

Files with missing lines	Patch %	Lines
message_ix/tools/add_year/__init__.py	2.8%	67 Missing ⚠️
message_ix/tests/tools/test_add_year.py	13.7%	25 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (d7452ee) and HEAD (91cdca4). Click for more details.

HEAD has 68 uploads less than BASE

Flag	BASE (d7452ee)	HEAD (91cdca4)
	71	3

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #494      +/-   ##
========================================
- Coverage   96.5%   26.5%   -70.0%     
========================================
  Files         60      60              
  Lines       5155    5172      +17     
========================================
- Hits        4975    1374    -3601     
- Misses       180    3798    +3618     

Files with missing lines	Coverage Δ
message_ix/tests/tools/test_add_year.py	17.8% <13.7%> (-78.2%)	⬇️
message_ix/tools/add_year/__init__.py	4.7% <2.8%> (-62.1%)	⬇️

... and 55 files with indirect coverage changes

[glatterf42]

Filtering out commits that were included in #703 already.