image: implement image getter and setter for dev purpose

[edgarriba]

This pull request includes several changes to the Image struct in the kornia-image crate, focusing on improving the API and adding new functionalities. The most important changes include updating the get_pixel method, adding a new set_pixel method, and renaming test functions to follow a consistent naming convention.

API Improvements:

• Updated the get_pixel method to return a reference to the pixel value instead of a copy.
• Added a new set_pixel method to allow setting the pixel value at given coordinates.

Documentation:

• Updated the documentation for the get_pixel method to recommend using the get method for better performance.

Tests:

• Renamed test functions to follow a consistent naming convention by prefixing them with test_. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
• Added new tests for the get_pixel and set_pixel methods.

Code Examples:

• Updated code examples in the ops.rs and threshold.rs files to reflect the changes in the get_pixel method's return type. [1] [2]

[johnnv1]

Lgtm