Allow single-variable BQM/QMs to be treated as variable labels

A frequent source of confusion in 0.10 is the distinction between `x` and `'x'` in
```python
>>> x = Binary('x')
```

The former is a `dimod.BinaryQuadraticModel` object, the latter is a string label of a variable. This leads to confusing behavior like https://github.com/dwavesystems/dimod/issues/921.

Would likely require https://github.com/dwavesystems/dimod/issues/966 and https://github.com/dwavesystems/dimod/issues/923.

The biggest challenge will be hunting down all of the locations that take a variable label and updating them. Though updating `dimod.Variables` would be an obvious place to start.

A good first step would be to raise a better error in some of the most common places it happens.