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This adds support for regularization of truncated diagonal
entries in `BoxConstrainedQuadraticFunctionalConstrainedLinearization`
If this is enabled, diagonal entries of truncated rows are set
to 1. In case of nested matrices this is applied recusively,
such that only the truncated scalar diagonal entries of nontrivial
diagonal blocks are modified. This can be handy if the truncated
linearized problem should be treated by a linear solver.
By default this is disabled, such that linear solvers for the
linearized problem have to be robust with respect to singular
problems due to truncation.

The extended TNNMG step requires these typenames for the LinearSolver constructor.
For the build-in linearizations the contrained types are the same as the unconstrained ones.

This is not true anymore: support for more general problems has been added.

Expressions in the false branch that cannot be instantiated
must be guared by the (template) identity. Suprisingly gcc
accepted this code (clang did not).

Make indentation consistent with editor hints

Now BoxConstrainedQuadraticFunctionalConstrainedLinearization
can be used with nested problems with block size >1.