BUG: Fetch previous iteration values from storage

Proposed changes

Ensures values for a previous_iteration variable are fetched from the stored values even if state is passed to Operator._evaluate.

What types of changes does this PR introduce to PorePy? Put an x in the boxes that apply.

[ ] Minor change (e.g., dependency bumps, broken links).
[x] 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).
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[ ] 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.