Allow measuring expectation values specified via eigvals + wires

[albi3ro]

Pennylane allows specifying measurements like ExpectationMP via a combination of wires + eigenvalues:

ExpectationMP(eigvals=np.array([-1.0, 1.0]), wires=qml.wires.Wires((0,1)))

While not user-facing, this can be produced internally, for example from:

>>> qml.expval(qml.X(0)).expand().circuit
[Hadamard(wires=[0]), expval(eigvals=[ 1. -1.], wires=[0])]

Trying to measure such an observable on lightning gives:

File [pennylane-lightning/pennylane_lightning/lightning_qubit/_measurements.py:145](pennylane-lightning/pennylane_lightning/lightning_qubit/_measurements.py#line=144), in LightningMeasurements.expval(self, measurementprocess)
    134     CSR_SparseHamiltonian = measurementprocess.obs.sparse_matrix(
    135         wire_order=list(range(self._qubit_state.num_wires))
    136     ).tocsr(copy=False)
    137     return self._measurement_lightning.expval(
    138         CSR_SparseHamiltonian.indptr,
    139         CSR_SparseHamiltonian.indices,
    140         CSR_SparseHamiltonian.data,
    141     )
    143 if (
    144     isinstance(measurementprocess.obs, (qml.ops.Hamiltonian, qml.Hermitian))
--> 145     or (measurementprocess.obs.arithmetic_depth > 0)
    146     or isinstance(measurementprocess.obs.name, List)
    147 ):
    148     ob_serialized = QuantumScriptSerializer(
    149         self._qubit_state.device_name, self.dtype == np.complex64
    150     )._ob(measurementprocess.obs)
    151     return self._measurement_lightning.expval(ob_serialized)

AttributeError: 'NoneType' object has no attribute 'arithmetic_depth'

I'm not too concerned about this, as default.qubit seems to not even really support this right now, and there's really no user-facing of creating this, but we should at least track it.