run for-loop in a parallel operation using "Tasks"

[ornwipa]

https://docs.microsoft.com/en-us/dotnet/standard/parallel-programming/how-to-write-a-simple-parallel-for-loop

[ornwipa]

https://dotnettutorials.net/lesson/parallel-for-method-csharp/ run parallel only when each serial is takes a long time, so only in cosine similarity within recommendation algorithm, but not when constructing user jagged array because each loop is quick but there is too many loops.

[ornwipa]

merge the revision locally