parallelize computation of horizons on multiple core

[ArcticSnow]

replace for loop of L162 with a pooling solution, spreading each horizon computation on different core in parallel

Resources:

• https://stackoverflow.com/questions/44118723/how-to-return-numpy-array-from-multiprocessing-pool-map
• https://programmerah.com/parallel-processing-in-python-pool-map-pool-starmap-pool-apply-196/