Add .tools.res_marg

[khaeru]

Partly addresses #275.

This PR does three things:

(1) Copies from message_data ssp_dev branch:

• File message_data/scenario_generation/reserve_margin/res_marg.py
• Last modified by commit iiasa/message_data@f3efc8b104044676434695aa461d26a7b20e5cd7
• message_single_country SSP_Dev_2023 branch contains an identical file.

(2) Adjusts the copied code to confirm, in a minimal way, to the code style:

• Docstring is correctly formatted.
• Modules and functions appear in the documentation.
• Code is type hinted.
• Tests exist, even if they are incomplete/fail.

The above two steps are mandatory for PRs related to #275.
Then, optionally:

(3) Improves the code.

• Add missing tests.
• Re-use existing utility code in message-ix-models, e.g. the CLI.

How to review

TBA

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 65.5%. Comparing base (dbd783e) to head (d8751c5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix_models/tools/res_marg.py	35.0%	13 Missing ⚠️
message_ix_models/tests/tools/test_res_marg.py	80.0%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (dbd783e) and HEAD (d8751c5). Click for more details.

HEAD has 59 uploads less than BASE

Flag	BASE (dbd783e)	HEAD (d8751c5)
	65	6

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #277      +/-   ##
========================================
- Coverage   77.6%   65.5%   -12.1%     
========================================
  Files        211     213       +2     
  Lines      16079   16109      +30     
========================================
- Hits       12481   10567    -1914     
- Misses      3598    5542    +1944     

Files with missing lines	Coverage Δ
message_ix_models/cli.py	93.4% <ø> (ø)
message_ix_models/tests/tools/test_res_marg.py	80.0% <80.0%> (ø)
message_ix_models/tools/res_marg.py	35.0% <35.0%> (ø)

... and 48 files with indirect coverage changes