Fix initial conditions

[vlipovac]

Closes #1344
Closes #1324

Proposed changes

Fixes the shortcomings discovered by @Yuriyzabegaev and implements the changes summarized in #1324, which was opened after #1236.

@keileg regarding our discussion about MRO issues and the usage of super, I have added a test which proofs that this is not the case for the initial condition framework. Hence it holds by extension for setting of variables, equations and BC (analogous usage of super), though not explicitly covered by tests.

Types of changes

• Minor change (e.g., dependency bumps, broken links).
• Bugfix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Testing (contribution related to testing of existing or new functionality).
• Documentation (contribution related to adding, improving, or fixing documentation).
• Maintenance (e.g., improve logic and performance, remove obsolete code).
• Other:

Checklist

• The documentation is up-to-date.
• Static typing is included in the update.
• This PR does not duplicate existing functionality.
• The update is covered by the test suite (including tests added in the PR).
• If new skipped tests have been introduced in this PR, pytest was run with the --run-skipped flag.

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[Yuriyzabegaev]

I really like the changes made here, good job @vlipovac! Please see my only concern in the review.

[keileg]

I post various comments here, but need to think more about the super-chain.

[Yuriyzabegaev]

Looks good to me, great job @vlipovac !

[keileg]

Nice work, @vlipovac! This makes much more sense now, and as you pointed out yesterday, the PR is much more managable now.

My main concerns have been addressed now, also regarding the super calls. I left some comments and suggestions. In my estimate, these should all be rather minor; please let me know if I underestimate something - then I misunderstood, and we should perhaps discuss before you dive into a not-small change.

[keileg]

I think we are good now. Thanks for the effort and patience, @vlipovac