Support initializing with `meta` dataframe in long format

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

In preparation for better integration with ixmp4, this PR supports passing a meta dataframe with model-scenario "index" as columns and/or in long-format as key-value columns.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (95ebfce) 94.7% compared to head (f937ea9) 94.7%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #801 +/- ## ===================================== Coverage 94.7% 94.7% ===================================== Files 62 62 Lines 5999 6013 +14 ===================================== + Hits 5683 5697 +14 Misses 316 316 ```

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