Moved chainer imports inside functions [no ci]

improved download logic to catch incomplete downloads

added github release download for parenchyma model

refactored to PEP8

tried to remove md5 for parenchyma model

more refactoring

worked, now have to add similar fix for all scripts

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.