QASM support for singly-controlled ControlledGates.

[mtweiden]

This PR adds support for outputting qasm of singly-controlled "standard" gates. Standard gates are those that have a defined qasm_name property.

Resolves #214

[edyounis]

Thanks for being quick with this fix! A couple of changes: I would override Gate.qasm_name or add an attribute to the class _qasm_name. This is because you are not changing how the entire qasm line gets printed.

Looking at the decoding, the cu1, cu2, cu3, cswap, c3x, and c4x are all the standard qasm2 identitifers that are handled this way. Does your implementation catch the cswap, c3x, and c4x?

I do like that your implementation handles multiple controls on gates nicely, but I worry this will also create a situation where we can encode something we can decode, e.g. ControlledGate(XGate(), 5). As annoying and limiting as this sounds, I think we should just check for the standard qasm ids, and print a helpful (or annoying but more specific) error in other cases.

Thanks again for putting this up!

[mtweiden]

This new version overrides the Gate.qasm_name property. There's a check now to ensure that only standard OpenQASM2.0 identifiers (or at least those in bqskit.ir.lang.qasm2.visitor.py) can be encoded.