Make it clear what to do if only using datashuttle locally (no data transfer, no central path)

[JoeZiminski]

This would include:

• adding some documentation to setup page, allowing central_path to be None
• grey out transfer page / raise easy to understand error on trying to use transfer functions if central_path is None.

This may be a little bit hairy because so far the design has not considered central_path to be None. We will need to check all instances in which it is used and raise errors appropriately. However, I think this should be fairly straightforward as only a few user-facing API rely on central_path.