Safeguard against using incompatible DB schemas #222
Description
As mentioned by @khaeru here, the Java backend code safeguards against using incompatible DB schema versions. That is to say, if a user creates and fills a DB using version a.b.c, then updated to version d.e.f, which includes changes to the DB model, the old ixmp code will prevent any errors resulting from that by either
- raising exceptions right away or
- performing a migration.
Either option would be fine for us, too, but we should add one of them.