Cannot mix hard links with external links -- need to check for this

[prjemian]

A question came up in the definitions repository about creating hard links within a master file to objects that are actually external linked from other files.

HDF5 will not permit this. The Python h5py package reported:

RuntimeError: Unable to create link (Interfile hard links are not allowed)

The NX5makelink() code in napi5.c is not defended against this situation. Suggest that additional code be added to check for the object being the target of an external file link before attempting to construct an HDF5 hard link. Could either quietly create an external file link or report an error.