Add support for regularizing truncated diagonal entries
This adds support for regularization of truncated diagonal
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.