Opower: Add gas sensors for utilities that report CCF

[tronikos]

Proposed change

Some utilities use CCF instead of THERM as the unit for GAS so handle them. The units are equivalent anyway. The sensors in Home Assistant have CCF as their unit.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes Delmarva uses CCF as the Unit of Measure for natural gas tronikos/opower#20
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
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• There is no commented out code in this PR.
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• Tests have been added to verify that the new code works.

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• Documentation added/updated for www.home-assistant.io

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