Adding ability to Notch 50Hz or 60Hz

Added code so that the user can enable a 50 Hz notch instead of a 60 Hz notch.  ~~There is no GUI button for it.   Should we do one?  Once the user does it once, it's not like they'll want to toggle back-and-forth.  I mean, they're only going to have 60Hz interference or 50Hz.  Should we really clutter the GUI with this extra button?~~

[EDIT: I went ahead and added the GUI button.  It works pretty well.]