function oscar0p33Path(catalog, attr) {
        var file = lookupOscar(catalog, attr.date);
        return file ? [OSCAR_PATH, file].join("/") : null;
    }
Returns the file name for the most recent OSCAR data layer to the specified date. If offset is non-zero,
the file name that many entries from the most recent is returned.

The result is undefined if there is no entry for the specified date and offset can be found.

UNDONE: the catalog object itself should encapsulate this logic. GFS can also be a "virtual" catalog, and
        provide a mechanism for eliminating the need for /data/weather/current/* files.

@param {Array} catalog array of file names, sorted and prefixed with yyyyMMdd. Last item is most recent.
@param {String} date string with format yyyy/MM/dd or "current"
@param {Number?} offset
@returns {String} file name