Move offset from columns to index in benchmark reported results

[ElizabethSantorellaQC]

When using benchmarks to check for correctness, we should only compare problems that have the same solution. For the binomial family, different offsets should yield different solutions, so we shouldn't be comparing offset and non-offset problems to each other. This can be achieved by moving 'offset' from the columns to the index of reported results, as Philipp did here:

#344 (comment)_