Support for user gate definitions

[thierry-martinez]

No description provided.

[matulni]

Thanks for the PR! I made some comments.

[matulni]

I suppose that this won't be necessary after #15 ?

[matulni]

I'm a bit confused by this variable: it's initialized to False and I don't see anywhere in the code where it could take a True value. I don't know how to test if it's a bug or not...

[matulni]

I think there seems to be a bug when trying to define custom gates without parameters. The following test

def test_gate_definition_without_arguments() -> None:
    """Test gate definition."""
    s = """
qubit[2] q;
gate fancyid a {
h a;
h a; }
fancyid q[0];
"""
    parser = OpenQASMParser()
    circuit = parser.parse_str(s)
    assert circuit.width == 2
    assert circuit.instruction == [H(0), H(0)]

raises

    @override
    def visitGateStatement(self, ctx: qasm3Parser.GateStatementContext) -> None:
        self.check_not_inside_gate_definition(ctx)
        name = ctx.Identifier().getText()  # type: ignore[no-untyped-call]
        param_identifiers = ctx.identifierList(0).Identifier()
>       qubit_identifiers = ctx.identifierList(1).Identifier()
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       AttributeError: 'NoneType' object has no attribute 'Identifier'

graphix_qasm_parser/parser.py:532: AttributeError

whereas the test

def test_gate_definition_with_arguments() -> None:
    """Test gate definition."""
    s = """
qubit[2] q;
gate fancyid(x) a {
h a;
h a; }
fancyid(0.2) q[0];
"""
    parser = OpenQASMParser()
    circuit = parser.parse_str(s)
    assert circuit.width == 2
    assert circuit.instruction == [H(0), H(0)]

passes.

OpenQASM supports custom gates without arguments, do you think it can be fixed in this PR ?