Clone between JDBCBackend vs. IXMP4Backend

[khaeru]

Ensure that scenarios can be cloned across platforms with different backends, especially from JDBCBackend to IXMP4Backend (the opposite direction is less critical).

This will be essential for migrating scenario data for message_ix and message-ix-models from our Oracle database servers to PostgreSQL servers/ixmp4.

Steps can include:

• Add tests that perform this clone.
• Check that the contents of the cloned scenario(s) are as expected.
• Check performance.
• Document usage.

[glatterf42]

Just noticing now that this is partially addressed by #610, which adds the functionality to clone from JDBCBackend to IXMP4Backend.
We could also say that that PR can close this issue since I don't think we need the functionality to clone from IXMP4Backend to JDBCBackend.