calcofi_mets — CF NetCDF releases

Versioning. Paths are named for the database release so a URL always answers "which release is this?". When a build is byte-identical to an earlier release the bytes are NOT duplicated — the release still gets a manifest and page, and the download points at the existing object. latest.txt holds the newest release.

CF scope. Fully CF: an underway series on a moving platform is a CF trajectory. One trajectory per cruise; time, latitude and longitude vary along the observation dimension rather than being fixed per instance.