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
object.