Add ability to merge serveral DQM or BQM objects

It would be useful to merge several BQM/DQMs.

At the moment one can do
```
bqm.update(other_bqm)
```
to merge two BQM objects.

We should add a `DQM.update` as well.

Further, it would be useful to combine many DQM/BQM together at relative weights. See https://github.com/dwavesystems/dimod/pull/784#issuecomment-769358918

One way to accomplish this would be a merge function, as suggested in https://github.com/dwavesystems/dimod/pull/784#issuecomment-769358918

Another would be to add a `weight` parameter to the `BQM.update()` and `DQM.update()` methods. Likely the function would end up calling the methods, but rolling them together would be nice.