Added IsingOperator to count_qubits.

[mstechly]

I've noticed that count_qubits in operator_utils doesn't handle IsingOperator and couldn't find a good reason why not, so I added it.

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[ncrubin]

@mstechly Thank you. Just a minor question and then we're good to go!

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