Extend IXMP4Backend

[khaeru]

Following #552 and related PRs for ixmp/message_ix v3.11, the goal for the next milestone (3.12; see iiasa/message_ix#939) includes “IXMP4Backend has feature parity with JDBCBackend.”

This issue is to accumulate/track atomic and collective issues and PRs for the tasks related to that goal.

• All code—particularly, all code in the message-ix-models package—that works with JDBCBackend also works with IXMP4Backend.
• Shared and remote databases are supported.
• Any discrepancy in behaviour or usage versus JDBCBackend is small and clearly documented.