calcofi_ctd-cast — 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: this dataset has one sampling level with many depths per event, which is exactly a CF profile, so it needs no extension beyond the standard.