add get_std() to simulators

[HaoTy]

Added native implementation calculating the standard deviation to all simulators. Tried my best to support as large a scale as possible and avoid unnecessary duplicate computations. Bonus: get_qaoa_objective() now can accept a sequence of strings as its objective (see https://github.com/jpmorganchase/jpmc-argonne-quantum-optimization/issues/169), so no more "expectation and overlap". Also, #42 is addressed but has not been tested yet.