Release 1.8.0: New Qiskit Backends

• new hybrid_qasm_simulator and hybrid_statevector_simulator backends for Qiskit that expose the hybrid Schrödinger-Feynman decision diagram-based simulator proposed in https://arxiv.org/abs/2105.07045
• new unitary_simulator backend for Qiskit that exposes the efficient decision diagram-based methods for constructing functional representations of quantum algorithms proposed in https://arxiv.org/abs/2103.08281