Refactor `nappy_api` to re-use code in the `nc_interface` and other modules rather than replicating code

[agstephens]

It appears the nappy_api.py module includes code that should really just be imported and exposed as an API.

We need to:

1. Remove duplicate code.
2. Decide on a clean way to expose the API.
3. Most complex code into other areas, such as the nc_interface package.