Update Qiskit Aer benchmarks for Qiskit Aer 0.4

[chriseclectic]

Updates Qiskit benchmarks (Closes #9)

• Fixes incorrect backend usage: Benchmarks are now run on the "qasm_simulator" with options set to force used of the dense statevector simulation method with certain optimizations disabled (OpenMP and qubit truncation)

• Updates native_execute function for change of function name in Aer 0.4

• Includes QCBM circuit benchmarks for Qiskit.

• Adds CUDA GPU backend for QCBM circuit benchmarks (requires separate install of qiskit-aer-gpu which is currently only available on Linux).

@Roger-luo Let me know if you have any issues running the updated benchmarks (particularly the GPU benchmarks) on your test system.

Updated circuit benchmark for qiskit run on a server with a P100 GPU (Note I didn't re-run any of the other simulator benchmarks they are plotted from repo data)

[Roger-luo]

Nice work, thanks! looks good to merge. I'll update the benchmark this weekend.

[chriseclectic]

@Roger-luo sounds good!