Big bug fix for the deletion of analyses tabs from the GUI!
This affects wxGTK users on Macs and Linux. Now the spectrum and relaxation data list GUI elements
have observer_register() methods, allowing the analysis delete_all() method to unregister all
analysis specific methods from the observer objects. This prevents queued wx calls to be made on
dead or non-existent GUI objects after the analysis has been deleted. wx.Yield() calls were not
sufficient to solve this issue.