concatenate should track which samples came from which response

**Application**
Sometimes we want to combine samplesets but still track which sampleset they originally came from.

**Proposed Solution**
Something like
```
>>> sampleset = dimod.concatenate([ss0, ss1], track_source=True) # each with one sample
>>> sampleset.record.source
[0, 1]
```

**Additional Context**
In general we are not supporting additional data vectors very well, for instance in concatenate itself. So need to somehow handle cases like:
```
dimod.concatenate([dimod.concatenate([ss0, ss1], ss2])
```