Nebuchadnezzar

-----

- Retrieve content from archive, rather than legacy.

- Make the collectiom version required for
``neb get [env] [colid] [col_version]``.
See https://github.com/Connexions/nebuchadnezzar/issues/54

- Warn and prompt if requested not most recently published version.

- Move all subcommand common options to the subcommands.
``neb [OPTIONS] get [OPTIONS] ...`` becomes ``neb get [OPTIONS``.
For example, ``neb -v get ...`` becomes ``neb get -v ...``.
See https://github.com/Connexions/nebuchadnezzar/issues/48

-----

- Fix the 'get' command to request a specific version of the completezip,
rather than rely on the 'latest' specifier, which has the issue of
requesting a cached completezip.
See https://github.com/Connexions/nebuchadnezzar/issues/44

-----

- Fix 'get' issue where the content exists but the completezip is not
available for download.
See https://github.com/Connexions/nebuchadnezzar/issues/28

-----

- Adjusted the publication api point in response to the api change in Press.

-----

- Add ``list`` command, to list individual environments defined
in configuration.

-----

- Clarify the error message produced when attempting to get content
that is already downloaded. This clarification is for when ``neb get``
would colide with an existing directory of the same name.