Fix serialisation of datasets instantiated from files

Summary

Ensure that datasets constructed using the files argument are successfully serialised/deserialised without losing file information.

Contents (#525)

Fixes

• Fix serialisation/deserialisation of datasets in Manifest when the datasets have been constructed using the files argument
• Use datafile cloud path in files field if available in Dataset.to_primitive
• Use Manifest.serialise instead of Manifest.to_primitive in Analysis.finalise

Dependencies

• Use twined=^0.5.1