[Nexus] Groups as data

[Mark Koennecke]

Hi,

before we christmas we had an e-mail discussion about how best to store NXpositioners. 
In one of that mails I stated that the ideal situation would be to store for example 
a rotation_angle where it belongs in the NeXus hierarchy and be able to open it as a 
group to see all the gory details. With current NeXus facilities this is not possible 
and thus we settled for storing NXpositioners somewhere in NXcollection and make 
links to the value where appropriate. 

Now, I had another idea: HDF does not allow to use groups as datasets. But if we use the 
NeXus-API it would be straightforward to implement something like this. 

Currently NXopendata returns with an error when called on a group.

NXopendata could be changed in the following way:
- If NXopendata finds that it is applied to a group, it first tries to locate a value SDS in the group 
  and opens this
- If the above fails, a value group attribute is searched for. This attribute should contain the 
  name of the  SDS to open as data for the group. If this exists open that SDS as data for the group
- Only if all of the above fails, return an error

There are caveats though: names need to be unique within groups. Those who use NeXus as 
HDF-5 with NeXus structures would have to worry about yet another convention. 

Is this an idea worth pursuing?


Best Regards,

   Mark Koennecke