Reading and writing the "/itn" group in C

Again, we start with an overview of relevant functions:

dm_h5_write_itn(h5_file_id, *ptr_itn_struct, *ptr_itn_array_struct, *error_string): Add a complex iterate to an already-opened HDF5 file.

dm_h5_itn_group_exists(h5_file_id): This routine determines if an "/itn" group exists.

dm_h5_read_itn_info(h5_file_id, *ptr_nx, *ptr_ny, *ptr_nz, *ptr_itn_struct,
error_string): This routine reads the itn_struct and the size of the ITN array from an already-opened HDF5 file.

dm_h5_read_itn(h5_file_id, *ptr_itn_array_struct, *error_string): This routine reads in itn_array_struct.

dm_h5_insert_itn_struct_members(datatype): This internal routine does the H5Tinsert() calls to build up the itn_struct variables