Add ECR gate

[vincentgierisch]

Add echoed-resonance (ECR) gate

[edyounis]

Awesome! Thanks for taking this upon yourself to add. This is very much a welcome addition, thanks also for getting the unitary endian ordering correct.

Besides the two minor points below, can you add a test that ensures we can read and write this gate properly to qasm. You may need to a GateDef in the qasm visitor module.

[edyounis]

Awesome! Just one more test to get this approved and merged in. Can you do a encode-decode-in-one test? As in create a circuit with an ECRGate, save to qasm with Circuit.to, then load that qasm in. I do expect that the resulting circuit will be composed of a CircuitGate with the decomposed gates, but it is good to check there are no crashes and that the input and output unitaries are the same.