Extend SSP argument options

[OFR-IIASA]

The mix-models --ssp= CLI option is used in the SSP 2024/ScenarioMIP workflows, but originally had only SSP1 through SSP3 as accepted values. This adds SSP4, SSP5, and LED.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.5%. Comparing base (d90c11c) to head (f6de7bb).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #233     +/-   ##
=======================================
- Coverage   66.5%   65.5%   -1.0%     
=======================================
  Files        204     204             
  Lines      15822   15822             
=======================================
- Hits       10526   10378    -148     
- Misses      5296    5444    +148     

Files with missing lines	Coverage Δ
message_ix_models/util/click.py	93.1% <ø> (ø)

... and 7 files with indirect coverage changes

[khaeru]

Thanks @OFR-IIASA. FYI @glatterf42, I will:

• Rebase.
• Complete the PR description and checklist.
• Ask for your review.

@macflo8, please let us know if you had made this change in a different way.

[glatterf42]

Looks good to me if the functions using --ssp also know how to handle the new values :)