Add Pearson correlation coefficient

[DjYash007]

Overview

This PR does the following:
Implement Pearson correlation coefficient in primary/correlation.rs.

Pearson correlation coefficient is a statistical measure of the strength and direction of the linear relationship between to variables.

Features:

• Reuses the existing covariance and std_deviation implementations
• Returns None when:
  • input lengths differ
  • fewer than two values are provided
  • either input has zero standard deviation
• Added tests for:
  • empty inputs
  • different input lengths
  • single element
  • zero standard deviation
  • smallest valid input
  • perfect positive correlation
  • perfect negative correlation
  • decimal values
  • multiple values
  • zero correlation

Essential Checklist

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Proof that changes are correct

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