Implement clone() in Python

[khaeru]

Do not merge; development code.

This PR is a sketch of how to (re-)implement clone() in Python, to address #423, iiasa/message_ix#254, and other issues.

Some of the requirements:

• clone() must not only work across 2 Platforms sharing the same Backend class (e.g. both JDBCBackend), but across different Backend classes, e.g. between a JDBCBacken and Add Python DB API backend #400 in either direction.

Things to check:

• That ixmp_source supports initializing and setting elements of 'variable' and 'equation'-type items.

  We've run into trouble with this in the past, e.g. working on Add Model.initialize #212, Add precautionary checks to Model.initialize_items #315.

How to review

To be added.

PR checklist

• Complete the code.
  • Extend item_set_elements() to cover variables and equations.
  • Complete the TODOs in clone().
• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.