Ensure writing a new datafile to the cloud does not require the file to already exist in the cloud

[cortadocodes]

The Datafile context manager is currently set up to do the following things:

• On enter:
  • Download an existing cloud object to a local file (via the local_path property)
  • Open it in the given file mode
• On exit:
  • Close the local copy of the file
  • If in a modification mode, overwrite the cloud object
  • If in a modification mode, optionally update the metadata of the cloud object

I'm pretty sure it also works for creating a new cloud object, but the code could be clearer and possibly tested better. This issue is about simplifying/clarifying the code and ensuring all use cases are tested.

Tasks:

• Clarify context manager code
• Write user stories
• Ensure all user stories are covered by tests