Add toString method to GlobalStats in ContestEstimator

[mmvpm]

Description

Add custom .toString() method to the GlobalStats class in ContestEstimator to display aggregated statistics:

Fixes #1593

Type of Change

• Minor bug fix (non-breaking small changes)

How Has This Been Tested?

Regression and integration tests

Please, provide regression or integration tests for your code changes. If you don't do that, the reviewer can and highly likely will reject the PR. It is the contributor's responsibility to provide such tests or to reason why they are missing.

Automated Testing

There is no need to test the ContestEstimator

Manual Scenario

Run ContestEstimator.main method and check the log

Checklist:

• The change followed the style guidelines of the UTBot project
• Self-review of the code is passed
• The change contains enough commentaries, particularly in hard-to-understand areas
• New documentation is provided or existed one is altered
• No new warnings
• All tests pass locally with my changes