Allow the conversion of time-of-flight to d-spacing

When loading a ISIS-GSAS file there is also the option of loading in a GSAS 
instrument file, which contains information that tell how time-of-flight 
relates to d-spacing as follows:

                     d=C*t

where 

   d = d-spacing
   t = time-of-flight
   C = a constant which can be calculated from the instrument file

So the enhancement is to extend the 'Transform x-axis' information 
panel to handle the conversion tof to d-spacing. This will require
that at some stage the user speficify the name of the instrument file
and this information is then stored with the Dataset.

An alternative would be that we insist that if the instrument file
is called something specific. For instance, if the GSAS file is bob.gss then 
the instrument file must be called bob.ins.

Original issue reported on code.google.com by anders.m...@stfc.ac.uk on 1 Jul 2010 at 4:55

arjeneh / jpowder

Allow the conversion of time-of-flight to d-spacing #23