Fix#4: Implemented variance function

[DjYash007]

Overview

1. This PR fixes or fixes part of Implement variance #4.
2. This PR implements the variance function for f64 slices in the primary module.

The implementation:

• Returns None for empty input and an edge case where sample has one element.
• Returns the variance.
• Uses existing mean module to calculate the mean and simple control flow logic for finding the solution.
• Also uses correct type references for avoiding compile time errors.

Essential Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• "Allow edits from maintainers" is checked. (See here for instructions on how to enable it.)
• The PR is made from a branch that's not called "main/master".

Proof that changes are correct

Verified locally with:

cargo test --test variance

Full cargo test currently also attempts to compile tests for median, variance, and std_deviation, which are separate unimplemented functions/issues and are not part of this PR.