ToDos before running MESSAGEix with ixmp4

[glatterf42]

This issue is supposed to give an overview of open ToDos before we can seriously test/run whole MESSAGEix workflows on ixmp4. To this end, I'll experiment with GitHub's new relations between issues :)

[mariyamali4]

This issue is still open but blocked, so does that mean we cannot run MESSAGEix with ixmp4 backend as yet?

[meksor]

Unfortunately not, but as I understand the last few pieces are falling into place.