[Nexus] some questions about NXdetector_module

[Eugen Wintersberger]

Hi folks 
  I am currently trying to improve the documentation of the `NXdetector`
class. What I am actually trying to do is to compute the spatial
location of a pixel in the laboratory frame for different experiment
configurations. My requirement is that `NXdetector` and
`NXdetector_module` should provide information to do this job. However,
there are some issues we may should think about.

issue 1.)
If all transformations involved (in particular `module_offset`) are
required to be translations, we can never deal with curved detectors
which might be very useful. I guess it thus would make sense to replace
`module_offset` my an instance of `NXtransformation`. The best solution
might be to make module_offset an instance of `NXtransformation` rather
than a field?

issue 2.) 
What exactly is the content of the `fast_pixel_direction` and
`slow_pixel_direction` field? Are this, for instance, the pixel center
positions along a detector axis?
As an example consider a 1D strip detector with 50um pixel size. Is 
fast_pixel_direction = [0,50,100,150,....] with units = um?

issue 3.)
If we want to handle 3D detector as well we maybe should ad a 
`medium_pixel_direction` field ;)

If anyone is interested in what I am doing try the issue_412 branch in
the git repository of the Nexus definitions. You find the new text in
the Nexus Design section in the manual.

regards 
Eugen

-- 
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DI. Dr. Eugen Wintersberger         
                                    
FS-EC                              
DESY                    
Notkestrasse 85                       
D-22607 Hamburg                    
Germany                            
                                   
E-Mail: eugen.wintersberger at desy.de
Telefon: +49-40-8998-1917          
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