[Nexus] Inverse geometry TOF spectrometer instrument definition

[Bull, MJ (Martyn)]

Hello Nexus people, 
 
Following the request from the NIAC for instrument editors to try writing a
definition using the current base classes, 
I've constructed a generic inverse geometry TOF machine template at:
 
http://www.neutron.anl.gov:8080/NeXus/88
<http://www.neutron.anl.gov:8080/NeXus/88> 
 
One of the interesting things about constructing the definition is the need
to decouple notions of logical neutron path in describing the instrument,
and instead to consider the instrument symmetry. This is a very different
approach to constructing instrument definition files for Monte Carlo
simulations.
 
The result is that analysers, collimators and filters are considered in much
the same way as banks of detectors. This leads to an implied order and
relationship between 'arms' of the instrument, which is currently not
handled by the definition. For example, it is implied that the collimator in
position 1 of the collimator bank array is associated with the analyser in
position 1 and the detector in position 1. Currently, if these positions are
mixed up, then it is only by reconstructing the instrument via the (x,y,z)
positions of each element, and visually examining the result that any sense
will be made of the description. 
 
I've also had to do some major hacking of the NXdetector class to make sense
of it, much like Tim Kelly's previous post to this list. 
 
Regards,
Martyn Bull, ISIS  
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