fix: allow multiple observables to solve from the same inline linsolve index

[AayushSabharwal]

The situation is that D(x) part of an inline linear SCC, so it turns into D(x) ~ linsolve(..)[i] and this goes into total_sub. Then, two observed equations are of the form

y ~ D(D(foo))
z ~ D(D(foo))

Where D(D(foo)) via total_sub resolves to D(x) and then linsolve(..)[i]. So multiple observed variables can be obtained from the same inline linear solve. This PR modifies unhack_system to handle that case.

The reproducer is from Dyad and is not something I'm able to MWE.

[baggepinnen]

once this is merged, can a release be made so that we can make a release of MBC? 🙏