Helper function around simulator's cheat function

[damiansteiger]

The simulator back-end already has a cheat() function to access the wavefunction and hence it is possible to calculate expectation values when using the simulator back-end. But it would be nice to have a few helper functions to make it easier to use and also an example in the tutorials.

[thomashaener]

Now, there is Simulator.get_expectation_value, which addresses this issue.