Plain filesystem storage

[jennydaman]

Summary

• CUBE is decoupled from Swift
• CUBE can use a filesystem as file storage

Motivation

Storing files in a directory on the host's filesystem is preferable to object storage in some cases:

• single-machine
• single-replica
• object storage not available
• use NFS for storage

Changes

• Minimized SwiftManager's public interface
• Created a new Python package, core.storage. core.swiftmanager was moved into core.storage
• StorageManager is an abstract interface implemented by SwiftManager
• Created new FilesystemManager implementation of StorageManager, which adapts pathlib to use a UNIX filesystem as the file storage service for CUBE
• tests were updated to replace mock.patch.object usage on SwiftManager with a custom context manager mock_storage which patches settings to use a temporary directory as the file storage service

How to configure: https://github.com/FNNDSC/ChRIS_ultron_backEnd/wiki/Storage-Backends