Simplify syntax to extract expectation values

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Describe the feature

Syntax for a simple hamiltonian is:

hamiltonian = spin.z(0)

results = cudaq.observe(kernel, hamiltonian)

exp_vals = results.expectation_z()

When we have a list hamiltonian, we have to run a for loop to extract results:

hamiltonian = [spin.z(0), spin.z(1)]

results = cudaq.observe(kernel, hamiltonian)

exp_vals = [results[i].expectation_z() for i in range(len(results))]

The for loop is cumbersome. Can we not allow expectation_z() to broadcast automatically over the results it is presented with?

Thanks team.

Start by tracing the cudaq.observe results and expectation_z() entry points shown in the issue, including both scalar and list Hamiltonian cases. Determine the intended broadcast behavior and add coverage for it in the relevant tests. Done means expectation_z() handles a list of results without a caller-written loop while preserving the scalar behavior.