Artemis: Refactor and Optimize Methods in Double Class

[mike-turintech]

This pull request refactors and optimizes several methods in the Double class to improve performance and readability. Key changes include:

1. sumSquare Method: Replaced nested loop with a direct formula, improving performance from O(n^2) to O(n).
2. sumTriangle Method: Simplified the calculation of triangular numbers using a direct formula, enhancing clarity and reducing complexity.
3. countPairs Method: Utilized a HashMap to count occurrences, improving performance from O(n^2) to O(n).
4. countDuplicates Method: Removed unnecessary nested loops, simplifying the logic and improving efficiency.

Overall, these changes make the code cleaner and more maintainable.