Reading and writing the "/spt" group in C

Again, we start with an overview of relevant functions:

dm_h5_write_spt(h5_file_id, *ptr_spt_struct, *ptr_spt_array_struct, *error_string): Add a support mask to an already-opened HDF5 file.

dm_h5_spt_group_exists(h5_file_id): This routine determines if an "/spt" group exists.

dm_h5_read_spt_info(h5_file_id, *ptr_nx, *ptr_ny, *ptr_nz, *ptr_spt_struct,
error_string): This routine reads the spt_struct and the size of the SPT array from an already-opened HDF5 file.

dm_h5_read_spt(h5_file_id, *ptr_spt_array_struct, *error_string): This routine reads in spt_array_struct.

dm_h5_insert_spt_struct_members(datatype): This internal routine does the H5Tinsert() calls to build up the spt_struct variables