ignore .DS_Store [no ci]

ignore .idea/

ignore all .npz files for future commits

Added option to segment the portal and hepatic veins (one class). Liver segmentation is now run in a separate Process, as TF is unable to clear the GPU manually - only when a session is killed, which is only when a Process is finished/killed.

added GPU support and bumped tensorflow to v2.4

improved argparse stuff