H5 string attributes should mimic what blimpy hdf_writer creates

[texadactyl]

Change

      DATATYPE  H5T_STRING {
         STRSIZE <strlen of a char * variable>;
         STRPAD H5T_STR_NULLTERM;
         CSET H5T_CSET_ASCII;
         CTYPE H5T_C_S1;
      }

to

         DATATYPE  H5T_STRING {
            STRSIZE H5T_VARIABLE;
            STRPAD H5T_STR_NULLTERM;
            CSET H5T_CSET_UTF8;
            CTYPE H5T_C_S1;
         }