Refactor multi-threaded case of parallelization functions
Refactor multi-threaded case of parallelization functions

- Extract multi-threaded setup logic into a generalized pthreadpool_parallelize function
- Call into pthreadpool_parallelize directly from tiled and 2+-dimensional functions

Maratyszcza/pthreadpool,Maratyszcza/pthreadpool,Maratyszcza/pthreadpool