hdf - pointer to a head HDF node
ctx - pointer that is passed to the HDFFILELOAD function when called
fileload - a HDFFILELOAD function
DESCRIPTION
hdf_register_fileload registers a fileload function that
overrides the built-in function. The built-in function
uses hdf_search_path and ne_file_load (based on stat/open/read)
to find and load the file on every hdf_read_file (including
#include). You can override this function if you wish to provide
other file search functions, or load the hdf file
from an in-memory cache, etc.