Rework how expectation values are defined

[JooNiv]

Rework the workflow around defining expectation values to make calculating other than the Pauli Z observables easier.

observables = {'z': [[0,1,2], [0,2]], 'x': [0,1]}

This could then be passed on as in the current version but would automatically calculate all the needed expectation values. Note that just like in the current implementation this would cause multiple experiment runs behind the scenes and would just streamline the user experience.

Needs reworking of how experiment circuits are generated, how experiment run results are stored, and how the expectation values are calculated in the end.