Allow user-defined types as a data type

[pvanlaake]

Topic for discussion

NetCDF version 4 allows for user-defined types (UDT), composed of elements of basic data types. UDTs are available for common programming constructs such as structs (compound UDT) and enum's (with vlen and opaque completing the list of currently defined UDTs).

One notable data collection where UDTs are already applied (and claiming CF-1.6 conformance) is the MODIS L3-binned data, where the UDTs inform the user of the storage arrangement of the ragged data.

More in general, a UDT allows one to group closely related data together, rather than define separate data variables. The u- and v-components of a vector field come to mind, but data variables referenced in formula_terms for a CV may equally be grouped together in this way.