DOC: note the target attribute does not work for external file links

[prjemian]

Fixes #841 as directed by NIAC2018, note the target attribute does not work for external file links.

[rayosborn]

The nexusformat API use the target attribute to define an external link, e.g.,

cspbbr3_300K['f1/data/data'] = NXlink(target='/entry/data/data', file='300K/f1.h5')

This is used to define the path within the external file to the linked data, but it is not, of course, stored in the external file itself. When nexusformat reads the external link, it creates an NXlink object that then contains this target, derived from the HDF5 external link definition, and this link resolves to the external file path when it is accessed. Since HDF5 disguises external links - it's actually quite difficult to find out that it is an external link - you need special calls to the h5py get function.

To summarize, our use of the target attribute is specific to nexusformat and is useful when using the API, but it is not a property of the stored parent file. It's not really a technical limitation - I think in a way, it shouldn't be part of the standard. It's up to the file writer whether they want the data to be external, but there is no difference to the innocent user. It's only an issue for any API the NIAC does or does not support. I don't know if this helps or not.

[prjemian]

@rayosborn I'm going to copy your comment to the #842, where the target attribute is to be fixed.