Fix for issue #14: Updated xml to store gain for uncalculated responses

If the user sets the dl gain but doesn't press calculate responses, it doesn't get saved to the stationXML. To fix this, I added two new methods. For the first method, when user does 'Write StationXML', it will check for stations with set gains, but no responses, and write the gains to the stationXML. For the second method, when the user does 'Load StationXML', it will check for stations with gain and no response and assign them to the stations in the inventory. Also added a popup window to let the user know the feedback of calculating responses.