return array in std_mean

[edgarriba]

This pull request includes changes to the crates/kornia-imgproc/src/core.rs file to improve the performance and readability of the std_mean function by using fixed-size arrays instead of vectors. Additionally, the test code has been updated to reflect these changes.

Changes to std_mean function:

• Modified the return type of std_mean from Vec<f64> to [f64; 3] for both standard deviation and mean values.
• Replaced the calculation of mean and variance from using vectors to using fixed-size arrays for better performance and readability.

Updates to test code:

• Updated the test code to use fixed-size arrays for expected standard deviation and mean values, improving consistency with the updated std_mean function.