Contour: fixes bug where setting maxField and/or minField in the GUI to null-string causes null pointer exception and a hangup.  If I am trying as a user to make just one contour level it seems at quick glance like I just need one value so nulling out the other field seems a perfectly natural thing the user might do.  The solution was to have the code see maxField=minField for the case where maxField=null, and vice versa.  If both fields are set to null, then both fields retain their original values.  For this last case perhaps a dialog could popup in the future the issue of setting at least one of the fields.