SampleSet.update or similar

**Application**
It would be useful to be able to add two overlapping sample sets together.

**Proposed Solution**
Something like
```
>>> new = sampleset.update(another_sampleset)
```
The case that needs be considered is recalculating the energies. We avoid that problem in #445 because the samples are disjoint. The 'easiest' approach would be to simply add the energies as in #445. Adding makes sense when the sample sets are generated by two different bqms, but that might be confusing for users who are updating based on generating the samples for a sub-bqm.