Simple initialisation to in-memory-platform

[danielhuppmann]

The pyam-ixmp4-tests use an in-memory-platform (see here), but this code seems complicated and can break when ixmp4 or dependences change (e.g., the alembic-version-check).

Would it make sense to have a simple class MemoryPlatform so that a user doesn't have to worry about transport and metadata?