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.
• [x] Update release notes.

[codecov[bot]]

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

```diff @@ Coverage Diff @@ ## main #852 +/- ## ======================================= - Coverage 95.6% 90.3% -5.4% ======================================= Files 46 47 +1 Lines 4333 4588 +255 ======================================= Hits 4143 4143 - Misses 190 445 +255 ``` | [Files](https://app.codecov.io/gh/iiasa/message_ix/pull/852?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa) | Coverage Δ | | |---|---|---| | [message\_ix/tools/add\_timeslice/add\_timeslice.py](https://app.codecov.io/gh/iiasa/message_ix/pull/852?src=pr&el=tree&filepath=message_ix%2Ftools%2Fadd_timeslice%2Fadd_timeslice.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa#diff-bWVzc2FnZV9peC90b29scy9hZGRfdGltZXNsaWNlL2FkZF90aW1lc2xpY2UucHk=) | `0.0% <0.0%> (ø)` | |