Optimize threshold_mut function with parallel iteration

[spartajet]

Replace serial iterators with par_iter_mut() for improved performance. Leverage Rayon for parallel processing of image pixels. Maintains same functionality while potentially reducing processing time.

[OrangeHoopla]

Would it make more sense to create a secondary function for this that uses par_iter so if users want to run this single threaded they have that option? Also the time to setup the thread pool might take more time then to just run this in a single threaded instance.

[spartajet]

You are right, I will create new function like threshold_mut_parallel