Make OneStepGridOperator work in nonoverlapping case

This is required for nonoverlapping computations. Just forward to the first contained GridOperator.

added backport label

added 102 commits

• 9d306209...79e7007a - 101 commits from branch master
• c85c5ddf - [onestep] Add make_consistent() to OneStepGridOperator

Compare with previous version

Rebased to have CI run again

merged

mentioned in commit f3a56eb0