Added h5_extend tiny example for reference
h5_extend optimisations:

 - setting file alignment to 4MB (good for GPFS)
 - setting chunk cache size to 4MB which is our chunk/test image size
 - flushing at the end of writing, before stopping timer
build system uncomment the MPI based build

This is now a plain old serial application.
Remeber to unset the CC and CXX environment variables.