Add option to disable "Refit" in Fit Grains Options Dialog

[psavery]

Currently, users don't have a choice to disable refit in the GUI. In the Fit Grains Options Dialog, it gives users an option to set refit scale settings, but not an option to disable refit altogether.

In the config, I believe the refit is set like refit: [2, 1]. If you disable refit, it should be set to refit: None instead.