Add a module for implementing (sub-annual) time slices

[glatterf42]

This PR supersedes iiasa/message-ix-models#197 as @khaeru suggested this tool might be better placed in message_ix.

@behnam-zakeri Please take a look at the changes. Several things come to mind before merging this:

• Use the existing logging functionality instead of printing to stdout
• Write a bit of documentation on how to use this tool (e.g. via a README.rst as was done for add_year)
• Write tests for this new functionality.

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PR checklist

• 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 0% with 255 lines in your changes missing coverage. Please review.

Project coverage is 90.3%. Comparing base (1125579) to head (baeb7f8).
Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix/tools/add_timeslice/add_timeslice.py	0.0%	255 Missing ⚠️

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Additional details and impacted files

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- Coverage   95.6%   90.3%   -5.4%     
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  Lines       4333    4588    +255     
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  Hits        4143    4143             
- Misses       190     445    +255     

Files with missing lines	Coverage Δ
message_ix/tools/add_timeslice/add_timeslice.py	0.0% <0.0%> (ø)

[glatterf42]

In the MESSAGE meeting just now, @behnam-zakeri mentioned that #680 might be related.

[behnam-zakeri]

@glatterf42 Related to the test, I have added an example of Westeros model via the Excel file, which is used by the code to create a Westeros scenario with time slices, exactly similar to the one we have in our published tutorials (Westeros_seasonality.ipynb). So, the only part missing will be to check if the output of these two models are the same or not.

1. Running the new code with input data from Westeros_baseline scenario and config data from Excel. [done]
2. Loading an already existing or running Westeros_Seasonality scenario (can be done easily).
3. Comparing the results of 1 and 2, e.g.:

• asserting if "time" related sets are the same ("time", "duration_time")
• asserting OBJ.
• asserting ACT of coal_ppl or wind_ppl in different times (winter or summer).