GSASII initial guess reflection peak fit

[DanielMurphy22]

The override unit cell values are used to match the reflections to the peaks. These reflections are generated in mantid. Saurabh would like a peak fit where a user can check how the initial guess of unit cell matches reflections with the raw data peaks. This "fitting" should probably act a little like the initial guess in the fit property browser.

This feature would allow the user to give the best initial guess to GSASII.

[DanielMurphy22]

When the loaded data table is on the GSASII tab, this preview function should allow the user to plot a data set and the markers for the pawley reflection positions which are calculated in the gsas2 model within mantid. This will help the user to choose good input parameters such as the override unit cell length which should alter the peak positions and help the user to pick a good initial guess for their refinement.