[from qiskit] Use dataclass for AlgorithmResult

[ElePT]

What should we add?

Now that the Python 3.6 support is dropped we can use dataclasses in Qiskit. Since our AlgorithmResult was essentially a dictionary replacement we implemented ourselves, should we switch to simply using dataclass as an algorithm result?

That would have advantages like

• much less overhead in writing algorithm results since no getters/setters are required (the docs can go into the class docstring)
• access to dataclass features such as easy conversion to dicts or easily creating immutable results

[ElePT]

Opened by @Cryoris
Original issue & discussion: Qiskit/qiskit#8437

[woodsp-ibm]

Also we should see whether we want/need to have something custom for a printable output - see comments in #58. The dataclass has a repr but potentially that may be viewed to have similar problems to the default in AlgorithmResult. Of course an end user can print any field they like - this was always a convenience to see everything - but everything may not scale well and a short result firm e.g. just the eigenvalue when thats what you are computing for, I can see too.