this patch writes index files for checkpoints if output_index is set. This is (only) useful if the checkpoints will be read in by a different number of processes than written. In that case, parsing the index files is faster than parsing the heavy data files.
The two patches provide read and write functionality. The attached C code creates index files from existing HDF5 files (not necessary checkpoints). It is a modified copy of hdf5_extract from the HDF5 thorn.