MTK indexing tests incorporated into Catalyst (dev) branch - potentially make a version for SciMLBase?

[TorkelE]

As previously described, I prepared a set of tests for indexinging of various problems, and it might make sense to adapt them to SciMLBase as well. They are now finished, and part of the Catalyst v14 dev branch. They can be found here: https://github.com/SciML/Catalyst.jl/blob/Catalyst_version_14/test/meta/mtk_structure_indexing.jl. There are still quite a few tests that are broken, these are marked as such.

For Catalyst they are quite easy, since a single model can be transformed to many types of problems quite straightforwardly. However, it should be possible to make a Catalyst-independent version without too much additional effort.

[ChrisRackauckas]

That would be really nice to have. SII integration is super important and we should test it at this level as well via a special downstream