Fix#13: Implement covariance function and add tests

[DjYash007]

Overview

1. This PR fixes or fixes part of Implement Covariance #13 : Implement covariance.
2. This PR does the following:
   Implemented covariance in primary/covariance.rs.
   Features:

• Reuses existing mean() implementation
• Returns None for empty inputs
• Returns None for inputs of different lengths
• Returns None for inputs with fewer than 2 values
• Added comprehensive test coverage including:
  • empty inputs
  • different lengths
  • single-element inputs
  • smallest valid input
  • multiple values
  • negative values
  • decimal values
  • positive covariance
  • negative covariance

Essential Checklist

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Proof that changes are correct

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