Fix #1: Implement Median function

[tannadevp]

Overview

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

Completed issue requirements:

• Created a new file : src/primary/median.rs
• Declared and re-exported median in src/primary/mod.rs
• Implemented the median function
• Added documentation comments
• Included usage examples in the documentation comments

Implementation:

• Returns None for empty input
• Returns the middle value for odd-length inputs
• Returns the average of the two middle values for even-length inputs
• Creates a sorted copy of the input so that the original slice remains unchanged
• Uses partial_cmp for ordering f64 values and rejects NaN values

The issue mentions primary/lib.rs, but this repository uses src/primary/mod.rs for the primary module, so median has been declared and re-exported there.

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:

1. cargo test --test median : passes for all the 11 median-specific tests
2. cargo test : attempts to compile for variance and std_devation as well, which are unimplemented functions/issues
3. cargo check : successfully executed
4. cargo fmt

Looking forward to your feedback. Please let me know if any changes are required. Thank you for reviewing!