API for converting notebooks between versions.
Convert a notebook node object to a specific version. Assumes that all the versions starting from 1 to the latest major X are implemented. In other words, there should never be a case where v1 v2 v3 v5 exist without a v4. Also assumes that all conversions can be made in one step increments between major versions and ignores minor revisions.
nb : NotebookNode
to_version : int