Add support for regularizing truncated diagonal entries

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.