Add result statistics on measurement outcomes in QAOAResult object

[vishal-ph]

Prerequisites

• [x] It does not lead to duplication.
• [x] It has not already been requested.

Is your feature request related to a problem? Please describe.

The QAOAResult object stores information regarding the QAOA optimization routine. This result object can be accessed post optimization through the QAOA workflow object via the following steps:

1. Define the problem qubo
2. Initialize QAOA workflow object q = QAOA()
3. Set QAOA properties via setter methods
4. Compile the object q.compile(qubo)
5. Run the optimization q.optimize()
6. Obtain the results of optimization as qaoa_result = q.result

Add a method under the QAOAResult class that computes some statistical quantities for the measurement outcomes.

Describe the solution you'd like

The QAOA object can store measurement outcomes (i.e. counts) from each circuit evaluation in the QAOAResult object.

• The method sorts the measurement outcomes (a dictionary) in the descending order of counts.
• It computes the mean and standard deviation for the optimized set of measurement outcomes.
• The function also optionally computes these quantities for all intermediate measurement outcomes during optimization if the user specifies.
• The method should raise an error if the user requires statistics on all intermediate measurement outcomes but hasn't specified saving them during optimization.

Additional context

An approach adding statistical tools to understand QAOA results better

[WingCode]

@vishal-ph I would like to take a stab at this issue. Could you assign it to me?

[vishal-ph]

@WingCode, sure! Thanks!

[TerraVenil]

@Q-lds please assign this issue to me. Thanks.

[Q-lds]

@TerraVenil, thanks for your contribution! :)