[Nexus] Correct way to specify multiple signals

[Pete Jemian]

On 1/11/2018 9:26 AM, Benjamin Watts wrote:
> NXdata was defined in order to allow a quick view of the data in order 
> to aid in sorting files.

Actually, NXdata was defined in order to allow a quick view of the data. 
(http://download.nexusformat.org/doc/html/motivations.html#simple-plotting)

I like the idea of being able to represent a view of the data that 
involves more than one signal.  Fitted data is a great use case of that. 
  The simple plot includes the measured data and the fitted data, with 
the possibility of error bars or such.

Mostly, NeXus has focused on storing raw data from the instrument.  As 
we continue to store processed data, we should consider how we might 
describe a simple plot of processed data.  The suggestions put forth 
here address that.

We should also avoid introducing unnecessary complexity.  Already, too 
many people view the myriad NeXus definitions not as options but instead 
as requirements.  Yet this should not blunt our efforts to describe data 
that we intend to use.

The suggestion for NXdata group attributes of auxiliary_signal and such 
is, I believe, the best approach presented so far.  We should work up an 
example or two (simple ones such as a plot showing SAS data and its fit 
or complex ones such as from the XRF community).

Pete