Optimized shallow copy for BQM

**Application**
It would be good for users to be able to decide which type of copy they want to use for the bqm.

**Proposed Solution**
To implement the `__copy__` method on the BQM object as defined in https://docs.python.org/3.7/library/copy.html. We may also want to add a `deep=False` optional parameter to the `BinaryQuadraticModel.copy` method.

**Additional Context**
The current implementation of `BinaryQuadraticModel.copy` uses python's `copy.deepcopy`. See #324, #326.

Potentially useful for https://github.com/dwavesystems/dwave-hybrid/issues/75.