Unable to import a scheme which happens to have the same id as an existing scheme

[as2388]

When a user attempts to import a code scheme via Dataset -> Load New Code Scheme from..., the import fails if that new dataset happens to have the same id as an existing dataset. It's not obvious to the user why this is a problem or what they can do about it.

I suggest when this happens we should tell the user which existing scheme is conflicting, then give them the following options:

• Update the conflicting scheme with the new one.
• Re-number the new scheme and its codes with a non-conflicting id.
• Cancel the import.

[as2388]

As a side effect of completing this task, users get a better work-around for #139 (by importing the same scheme over and over)