Qmin, Qmax calculations in refl_gui

[OwenArnold]

This line:

https://github.com/mantidproject/mantid/blob/master/Code/Mantid/scripts/Interface/ui/reflectometer/refl_gui.py#L982

Is performing the same unit conversion defined for Momentum Transfer here : http://www.mantidproject.org/Unit_Factory

We are now very close to stripping all calculations out of the refl_gui. It would be good to create a temp SingleValue workspace for this
lambda value, and then run ConvertUnits rather than keeping the fragile calculation above. Ask Owen Arnold if this doesn't make sense. Be careful that you
can reproduce exactly the same result. You will need to do this sort of thing in the new gui anyway.

Need to ask about the following one. Create a separate ticket for this one when done with the above.

https://github.com/mantidproject/mantid/blob/master/Code/Mantid/scripts/Interface/ui/reflectometer/refl_gui.py#L757

[OwenArnold]

This issue was originally trac ticket 10217