Speed up OCR - Githubissues

Defined a central ThreadPool that is used everywhere for better resource allocation.
Utilized the ThreadPool in read_descr for increased parallelism.
Refactored ocr.read to allow multiple threads by introducing an APILoader.
Fixed some tests to allow them to run independently of the current working directory.

Speeds up read_descr by about 15% on my desktop

aeon0 / d4lf