move implementation details of linear correction out of TNNMGStep
This simplifies the `iterate()` method such that it fits on a single screen.
| Status | Name | Job ID | Coverage | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Test | |||||||||
| passed | dune:git--clang |
#9625
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00:04:15
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| passed | dune:git--gcc |
#9626
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00:03:54
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