Add direct CAS-to-CAS import for _casbaseddirectory.py

Background

Direct CAS-to-CAS import was removed from !481 (merged) since the same result can be performed via a normal filing system, although we expect direct CAS-to-CAS import to be quicker.

Task description

• Alter CASBasedDirectory.import_files so it recognises an import from another CASBasedDirectory and performs a direct import without using any file systems.
• Test performance with a suitably large dataset (e.g. Freedesktop-SDK)

Acceptance Criteria

CAS-to-CAS import should produce identical results to importing via the file system and should be faster. If it doesn't show any increase in speed we shouldn't merge it.