OpenQASM AST for DelayInstruction/QuantumBarrier `qubits` attribute name is misleading

[stephteo]

The AST for DelayInstruction and QuantumBarrier defines a qubits attribute, but in oqpy we also use them for frames so the attribute naming is misleading. The typing is not an issue here.

@dataclass
class DelayInstruction(QuantumStatement):
    """
    Delay instruction

    Example::

        delay[start_stretch] $0;
    """

    duration: Expression
    qubits: List[Union[IndexedIdentifier, Identifier]]

@dataclass
class QuantumBarrier(QuantumStatement):
    """
    A quantum barrier instruction

    Example::

        barrier q;
    """

    qubits: List[Expression]