netCDF LD prefixing

NetCDF-LD elements (netCDF attribute names and string values) have the form: elem = [prefix__]suffix

where prefix and suffix are alphanumeric strings starting with letters and containing no whitespace. ([] indicates an optional part.)

A prefix is defined by a netCDF string attribute with a name composed of a prefix followed by a double-underscore and a value equal to the leading part of a URI. A complete URI is composed by concatenating the prefix value and the suffix value. Prefix definitions can be created as attributes on a group that acts as a container.