Add result statistics on measurement outcomes in QAOAResult object

[TerraVenil]

Description

This PR is for solving issue #235.

Checklist

• I have performed a self-review of my code.
• I have commented my code and used numpy-style docstrings
• I have made corresponding updates to the documentation.
• My changes generate no new warnings
• I have added/updated tests to make sure bugfix/feature works.
• New and existing unit tests pass locally with my changes.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

test_qaoa_results_calculate_statistics_full, test_qaoa_results_calculate_statistics_without_intermediate, test_qaoa_results_calculate_statistics_raise_value_error.

[TerraVenil]

Hi, @vishal-ph . Just let me know if we want to handle analytical_simulator in a special way.

[Q-lds]

@TerraVenil, very nice contribution! I just left two minor comments/suggestions and then we are good to merge :)

[edit] Make sure you write a message under #235, so that I can assign the issue to you :)