Add verification in examples/gas/run_gas_obrf.sh

Originally reported by: **Yung-Yu Chen (BitBucket: [yungyuc](http://bitbucket.org/yungyuc), GitHub: [yungyuc](http://github.com/yungyuc))**

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Compare the simulated results with the analytical solution.

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- Bitbucket: https://bitbucket.org/solvcon/solvcon/issue/108
