Zb/illico fixes

[zboldyga]

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[zboldyga]

@ilan-gold this is a work in progress but opening a draft to keep you in the loop. This PR is scoped only to fixes to the current illico integration. The 3 commits here correspond with unique fixes

As for the groupby change, I want to double-check this and refine a bit more, but the prior version has dependencies on the illico internal api/decisions, and is slower than the proposed approach (~a few seconds runtime for some datasets I tested, could be more for bigger ones).

[ilan-gold]

Nice this looks like a great start (and a big cleanup over what I had!)

[zboldyga]

@ilan-gold OK I reviewed carefully and I feel this is sufficient / complete for the scope of this PR.

Kind of note to self here, but overall if/when illico is more tightly integrated the whole flow from X to final stats can involve fewer operations. I suspect the unstack step won't be needed (the _to_iter function will be removed later, and it's tests as well).

[ilan-gold]

@zboldyga This looks good overall. Just to understand, was this responsible for the performance difference?