Parallelize loops in post-processing to improve performance

[timmintam]

What should we add?

Many of the loops in the post-processing stage are sequential but could be parallelized. Parallel processing could significantly boost the performance, especially for experiments with lots of shots or observables with many pauli terms.

Suggested Improvements:

• Identify bottlenecks in post-processing loops.
• Evaluate feasibility of parallel execution.