[Nexus] NXroot attribute

[V. Armando Sole]

I think an attribute to indicate the HDF5 version is convenient.

Concerning the software used to write the file, there are so many 
possibilities that perhaps we should foresee an attribute for it because 
we cannot expect an attribute for each writer. If the application 
writing the file decides to fill it, always just for informative 
purposes, with nexusformat, the application name, h5py X.X.X, or 
whatever at least everybody knows where to look for the information.

In any case, the most important thing is to know the HDF5 version used 
to write the file.

Armando