Add CoC (Cost of Capital) feature parameterized by international climate finance level

[yiyi1991]

This PR adds the CoC (Cost of Capital) feature for issue #428 .

• introduce international climate finance level as a key external parameter
• currently only covers power generation technologies (to be extended to energy demand technologies)
• examines how the weighted average cost of capital (WACC) of power generation technologies dynamically reacts to the (cumulative) international climate finance (ICF)
• projects future WACC
• separates the cost of capital part from the overall investment cost
• applies to any R12 starting scenarios
• can be run by CLI commands

TODO
(key functions there, scenario runs finished, slowly clearing up...)

• add documentation
• fix hard-coded indicator generation in WACC projection
• add starting scenario and output scenario in the CLI options
• add tests
• one more regression package linearmodels needs to be installed in user's environment

How to review

• Read the diff and note that the CI checks all pass.
• Run mix-models coc run --from="base" "coc reported" --dry-run and check if the workflow successfully generated.
• xxx
• If the documentation makes sense.
• Ensure that changes/additions are self-documenting, i.e. that another developer (someone like the reviewer) will be able to understand what the code does in the future.

PR checklist

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

[codecov]

Codecov Report

❌ Patch coverage is 2.40770% with 608 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.0%. Comparing base (cb47229) to head (362521b).
⚠️ Report is 73 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix_models/project/investment/workflow.py	0.0%	196 Missing ⚠️
...sage_ix_models/project/investment/2_Generate_CF.py	0.0%	107 Missing ⚠️
..._ix_models/project/investment/3_WACC_Projection.py	0.0%	106 Missing ⚠️
...x_models/project/investment/4_Generate_inv_cost.py	0.0%	88 Missing ⚠️
...ge_ix_models/project/investment/1_FE_Regression.py	0.0%	64 Missing ⚠️
...ssage_ix_models/project/investment/Region_utils.py	0.0%	24 Missing ⚠️
message_ix_models/project/investment/cli.py	39.4%	23 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #455     +/-   ##
=======================================
- Coverage   75.6%   73.0%   -2.6%     
=======================================
  Files        278     285      +7     
  Lines      22740   23363    +623     
=======================================
- Hits       17201   17067    -134     
- Misses      5539    6296    +757     

Files with missing lines	Coverage Δ
message_ix_models/cli.py	93.4% <ø> (ø)
message_ix_models/project/investment/cli.py	39.4% <39.4%> (ø)
...ssage_ix_models/project/investment/Region_utils.py	0.0% <0.0%> (ø)
...ge_ix_models/project/investment/1_FE_Regression.py	0.0% <0.0%> (ø)
...x_models/project/investment/4_Generate_inv_cost.py	0.0% <0.0%> (ø)
..._ix_models/project/investment/3_WACC_Projection.py	0.0% <0.0%> (ø)
...sage_ix_models/project/investment/2_Generate_CF.py	0.0% <0.0%> (ø)
message_ix_models/project/investment/workflow.py	0.0% <0.0%> (ø)

... and 7 files with indirect coverage changes

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[khaeru]

This entire file can be dropped by using existing code:

• .util.pycountry
  • This project should extend the COUNTRY_NAME mapping in that module (instead of defining manual_map, below), then call the function iso_3166_alpha_3().
• .model.structure.
  • This project can call get_codelist("node/R12") and then derive the structural info necessary from the code list.

Please ask if there is any lack of clarity or examples in existing code, or if it's not evident how to make the substitution.

[khaeru]

I feel the top-level module name message_ix_models.project.investment might lead to confusion in the future if there are multiple projects that deal with investment (seems likely). Some ways to disambiguate would be to add a two-digit year to the module name:

• message_ix_models.project.coc_25
• message_ix_models.project.investment_25

The choice of a CLI entry point mix-models coc points to the former.

[khaeru]

Please keep the list sorted.