Tie Heights option for ConvFit

[robertapplin]

Requested by: Spencer

Description:
Spencer has requested that we add an option to ConvFit to make it easier to tie the Heights of the two lorentzians when 2 lorentzians are selected. There is a very similar option in IqtFit which is used called "Tie Intensities", except it ties different parameters.

Seeing as there is already a precident for this sort of thing, I think it could be a worthwhile addition.

Maybe the code behind this option can be made more generic so it is easier to add similar options in the future if required.

Acceptance Criteria

• An option to "Tie Heights" exists in the ConvFit tab of QENS Fitting
• The code is generic, maybe reusing code used for the "Tie Intensities" option in IqtFit