Flatten nested controls in ControlledViaAnd

[anurudhp]

Override ControlledViaAnd.get_ctrl_system so that ControlledViaAnd(bloq, spec1).controlled(spec2) now returns ControlledViaAnd(bloq, spec) where spec is the sequence of spec1, spec2.

[mpharrigan]

add a test and ideally a little demo to the doc notebook.

also can you add a description to the PR

[mpharrigan]

where is this called? doesn't the call site need to be updated?

[anurudhp]

It is only used in one location, to convert cirq style controls to CtrlSpec:

Qualtran/qualtran/_infra/gate_with_registers.py

Lines 499 to 501 in db8b993

	ctrl_spec = self._get_ctrl_spec(
	num_controls, control_values, control_qid_shape, ctrl_spec=ctrl_spec
	)

I made this a class method so that I can call it in #1348 (as the code doesn't depend on the instance).

[mpharrigan]

can you please update that existing call in gate_with_registers.py to call it as a class method, i.e. GateWithRegisters._get_ctrl_spec instead of self.

[anurudhp]

done