Add TeixeiraWaterIqt to IqtFit

[robertapplin]

Description of work

This PR adds the TeixeiraWaterIqt fit function to the IqtFit tab of the inelastic QENS Fitting interface. It makes sure that the q values for the fit function are updated when changing the plotted spectrum.

Fixes #36502

To test:

1. Open Inelastic QENS Fitting interface
2. Go to IqtFit
3. Load the following data
4. Select the TeixeiraWaterIqt option in Fit Type
5. It should have three parameters
6. Select "See full function"
7. Check that the Q value updates whenever you change the plotted spectrum
8. Click "Run" and wait
9. It should successfully perform a fit

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[adriazalvarez]

Thanks for implementing the Teixeira water on IqtFit. I have tested it and it is easy to make fix now from the tab.
But even if the fit is successful, it is not very good. I have found that compositing with a LinearBackground function improves a lot the Teixeira fitting. I think the linear background fitting function would be a good addition to the background fitting functions on this property browser (I know it can be done with the full function browser, but it would be better to have it in the simple function menu). But that can be done on a separate issue if you agree.

[robertapplin]

I think the linear background fitting function would be a good addition to the background fitting functions on this property browser (I know it can be done with the full function browser, but it would be better to have it in the simple function menu). But that can be done on a separate issue if you agree.

Thanks for the suggestion, I think it would be good to check with Spencer, and then done as part of a separate issue if he agrees

[adriazalvarez]

Thanks for addressing my comments. I'm happy to approve, it is a nice addition to IqtFit Tab.

[SpencerHowells]

@robertapplin @adriazalvarez The IqtFit gui already has the Flatbackground option. You can then Tie intensities s\o that they sum to 1.