Improvements to 'transfer_data.py' utility

[pjbriggs]

PR implements a number of improvements to the transfer_data.py utility:

• Adds a set of unit tests
• Implements a new "job handler" class to reduce boilerplate code for performing transfer operations via scheduler jobs
• Refactors internals of the utility for clarity
• Improves reporting of operations and summarising of final dataset
• Writes transfer summary to file in the working directory
• Adds --dry-run option