Linearization: Allow to hand over 'ignore' after object has been created

Previously the 'ignore' field could only be set in the constructor.
But the 'ignore' field is part of the iteration step, and as a
consequence there was no way to have a Linearization with non-standard
state.  For the brittle-fracture code I want to set
gradientAwareTruncation in BCQFConstrainedLinearization, for which
the Linearization object needs to be constructed before the TNNMGStep
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