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.

How to review

Check the above boxes plus:

• Read the diff and note that the CI checks all pass.
• Run a specific code snippet or command and check the output.
• Build the documentation and look at a certain page.
• 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 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).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #852     +/-   ##
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- Coverage   95.6%   90.3%   -5.4%     
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  Files         46      47      +1     
  Lines       4333    4588    +255     
=======================================
  Hits        4143    4143             
- Misses       190     445    +255     

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