add the pow_exp kernel and tested

[SAbdulah]

This pull request pertains to add the power exponential kernel to ExaGeoStatCPP.

[mahmoudElkarargyBS]

Fantastic job! I'm delighted to witness the seamless integration of your contribution into our codebase. To enhance the robustness of our implementation, could you take an additional step by incorporating a test file for your kernel? This step is crucial in ensuring that the implemented kernel undergoes thorough testing and produces the anticipated results.

You can replicate any of the existing tests from another kernel by following these steps:

Duplicate the entire file.
Integrate it into the CMake file directory.
Modify the values such as N, dts, and theta's to align with your implementation.
Specify the required test conditions to validate that the output aligns with your expectations.
This will not only validate the correctness of your kernel but also contribute to the overall reliability of our codebase. Thank you for your dedication to maintaining code quality.

[mahmoudElkarargyBS]

Also, Don't forget to add it to the Documentation as a new kernel we support.