move implementation details of linear correction out of TNNMGStep

This simplifies the `iterate()` method such that it fits on a single
screen.
2 jobs for cleanup/move-details-of-linear-correction-out-of-tnnmgstep in 4 minutes and 15 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed dune:git--clang #9625

00:04:15

passed dune:git--gcc #9626

00:03:54