TNNMGStep: Allow to hand over a Linearization object

For the brittle-fracture code I want to set gradientAwareTrunction
in BCQFConstrainedLinearization, for which the Linearization object
needs to be constructed before the TNNMGStep object.

As a consequence, TNNMGStep now needs to be able to hand over the
'ignore' field to an already-constructed Linearization.
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