Migrate .report.legacy code from message_data dev branch

[khaeru]

This branch/PR exists to record the contents of message_data.tools.post_processing from the dev branch of message_data as of this moment; specifically iiasa/message_data@c22563f754a0e3802deb477848a30832e0fae0df.

• In Migrate legacy reporting #159, message_ix_models.report.legacy was created by migrating code from a different message_data branch, ssp_dev.
• This PR overwrites those files with the corresponding files from iiasa/message_data@c22563f754a0e3802deb477848a30832e0fae0df with zero changes.
• ssp_dev and dev are divergent branches developed from the main branch of message_data. The change here thus conflict in many ways.
  • It is likely some of them are obviated or superseded by changes made on that ssp_dev branch, either prior to or after the point where Migrate legacy reporting #159 was migrated, or adjustments and clean-ups in that other PR.
  • For example, changes that appear as deletions here may be content that was added on message_data ssp_dev after it diverged from main, but not added on dev. These should be ignored/not merged.
• Consequently this branch should not be merged as-is, or probably ever. Instead, useful pieces of the diff/commits can be cherry-picked and included via other PR(s)/branches.
• When everything useful from the branch is collected, the branch can be archived.

Also related: #4, #27

How to review

N/A

PR checklist

• Continuous integration checks all ✅ N/A, not to be merged.
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.