[FEAT] Implemented covariance calculation Closes #48

[jiyo4476]

• Covariance Test cases
• NDArray::cov function in numpower to expose to PHP
• Covariance implementation without Hstack since not in branch

Submission Checklist:

Due to the inherent complexity of this library, we created this checklist to remind everyone of the essential steps to have an MR approved depending on the type of change that is made. You can delete this.

• [ X ] Have you followed the guidelines in our Contributing document?
• [ X ] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ X ] Does your submission pass tests with ZEND_ALLOC enabled? export USE_ZEND_ALLOC=1 && make test
• [ X ] Does your submission pass tests with ZEND_ALLOC disabled? export USE_ZEND_ALLOC=0 && make test

Change to methods and operations

• [ X ] Have you verified that your change does not break backwards compatibility?
• [ X ] Have you updated the operation(s) tests for your changes, as applicable?
• [ ] Optional: Have your changes also been tested on the GPU? If you don't have a GPU available, you'll need to wait for a community member to perform the approval with a GPU. This only applies to changes that can affect GPU functionality.
• [ ] Optional: Have your changes also been tested on the GPU with the NDARRAY_VCHECK option enabled and no VRAM memory leaks were displayed? export NDARRAY_VCHECK=1 && make test

[SkibidiProduction]

so, will this PR be merged?