Add Weighted Moving Average

[navdeep-G]

• Implement WeightedMovingAverage class with linear weight distribution
• Add comprehensive unit tests covering edge cases and mathematical correctness
• Support configurable period with validation for positive integers
• Return null values for insufficient data points (period-1 elements)
• Include sliding window functionality with proper weight calculations
• Test coverage includes: constructor validation, edge cases, precision,
  trend following, reset functionality, and mathematical accuracy

The WMA assigns higher weights to more recent values, providing better trend responsiveness compared to Simple Moving Average while maintaining smoothing properties. Useful for financial analysis, inventory forecasting, and time series trend detection.