Support Zenodo versioning

This is the implementation of a feature requested in #15 (closed) .

It allows to create a new version from an existing Zenodo record instead of creating an independent record.

The way it works:

An option --zenodo-version-update has been added to prepare_zenodo.py:

• If codemeta, a new version will be created on Zenodo if a Zenodo ID is found in the CodeMeta file.
• If none (default), the previous behavior will apply, i.e. a new independent record is created on Zenodo.
• Any other value will be treated as a Zenodo ID, and a new version will be created from this ID.

Note: This changes the behavior of FACILE-RS in existing workflows. Should we set the default as "none"?

Edit: I've set "none" as default, I think it's better as it can raise errors in existing workflows if the identifier in the CodeMeta file has not been published.