Bug with Python API with no TNQVM install

[amccaskey]

If no TNQVM plugins are installed, python api defaults to vqe-dummy accelerator even when the user has specified xacc.setOption('accelerator','blah'). Reported by @eugescu

Easy fix:

	if (!xacc::optionExists("accelerator")) {
		xacc::setAccelerator("vqe-dummy");
		// Set the default Accelerator to TNQVM
		if (xacc::hasAccelerator("tnqvm")) {
			xacc::setAccelerator("tnqvm");
		}
	}