Add support for transient HDF5 datatypes

[ZedThree]

As raised in #267 and #2650, with HDF5 it is possible to store user-defined datatypes within a variable, for example:

HDF5 "test.h5" {
GROUP "/" {
   DATASET "x" {
      DATATYPE  H5T_COMPOUND {
         H5T_IEEE_F64LE "r";
         H5T_IEEE_F64LE "i";
      }
      DATASPACE  SIMPLE { ( 1 ) / ( 1 ) }
      DATA {
      (0): {
            1,
            0
         }
      }
   }
}
}

or

HDF5 "test.nc" {
GROUP "/" {
   DATASET "foo" {
      DATATYPE  H5T_ENUM {
         H5T_STD_I8LE;
         "FALSE"            0;
         "TRUE"             1;
      }
      DATASPACE  SCALAR
      DATA {
      (0): TRUE
      }
   }
}
}

NetCDF can already read these datatypes if they have been committed to the file with a name (just in the root group, or in any group?), so presumably it's just the type-lookup logic that needs to be modified?

[edwardhartnett]

Yes I believe you are correct. This is a the result of a mistake on my part, I just did not understand that these type definitions could exist.

Indeed the answer is to modify the metadata reading functions to spot these types and read them.

[DennisHeimbigner]

Does this require any specific HDF5 superblock number? We need to make sure that
netcdf-java can access such types.

[ZedThree]

Nope, no requirement on the superblock version, this has been in HDF5 since at least 1.6, more than 15 years.