Keeping constraint labels when converting cqm to bqm

**Application**
Currently, when one converts a cqm to bqm, labels of the constraints are discarded and generated from scratch, resulting in slack variables with the new label. Also, in case a warning is given, it is given with the new label making it impossible to understand for which constraint it is about.

**Proposed Solution**
Instead of generating them from scratch, old labels can be kept, which will make easier issues mentioned above.