Add .model.magpie, .project.justmip

[khaeru]

This PR adds (currently only placeholders for) two new modules:

1. .model.magpie should contain generic code for setting up and parametrizing the linked MESSAGE-MAgPIE model combination.
2. .project.justmip should contain code to use this generic capability in the way(s) required as part of the JustMIP project. Note that this module also may contain much other code related to the project, but for the present PR we only focus on separating out the usage from the capability w.r.t. MAgPIE.

These will eventually hold the files described in this Slack thread. Those files will be added in this and other PRs.

To complete:

• Complete the JustMIP project description page.
• Add the identified files.
• Identify and set code owners for the added files/directories.

How to review

TBA

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 76.7%. Comparing base (dbd783e) to head (b51f015).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #274     +/-   ##
=======================================
- Coverage   77.6%   76.7%   -1.0%     
=======================================
  Files        211     211             
  Lines      16079   16079             
=======================================
- Hits       12481   12333    -148     
- Misses      3598    3746    +148     

see 7 files with indirect coverage changes