Clone across platforms (database instances) not operational about ixmp

[khaeru]

I've just run into this. @zikolach does this require anything special to be changed/added in the Java code? If not, I can take a stab at it.

[zikolach]

@khaeru feel free to contribute. I wouldn't exclude the case that something doesn't work as expected on Java side as well. There are some tests on both sides (java and python) which seem to work. And also latest ixmp.jar might have this indirectly touched (e.g. in commit a782e2edffeaa6fb4bbb7cbe2fbcd0c79d06e0cd in ixmp_source repo)

[zikolach]

Related fix https://github.com/iiasa/ixmp_source/pull/141/commits/d5229563430b64a16d005dc477a09628f0591a12 was merged as part of PR iiasa/ixmp_source#141

[danielhuppmann]

Please look at danielhuppmann@15ddb5d which extends the existing (incorrectly passing) integration tests to make them fail - as they should, because the current implementation does not actually clone a Scenario across platforms. Instead it only clones the Scenario within the same platform and just overwrites the Python-internal mp arg.

[zikolach]

@danielhuppmann @khaeru there was missing first parameter in clone method (which is needed to cross-db cloning)