Store pauli strings as a std::map or std::unordered_map in PauliOp

[stand-by]

How about storing pauli_strings as a std::map or std::unordered_map instead of current std::vector?
That way it would be faster to resolve duplicated strings we end up with in several operations we have implemented for pauli op. We also wouldn't have to overload the interface with questioning user whether they want to compress or set dedupe=true.