Added third state to nullable checkboxes

VECTO3GUI2020/Views/Multistage/CustomControls/MultistageParameterCheckBoxView.xaml

@@ -37,7 +37,7 @@
                       Visibility="{Binding ShowConsolidatedData, Converter={StaticResource BooleanToVisibilityConverter}}">
             </CheckBox>
 
-            <CheckBox x:Name="checkBox1" Grid.Column="3" VerticalAlignment="Center" HorizontalAlignment="Center" 
+            <CheckBox x:Name="checkBox1" Grid.Column="3" VerticalAlignment="Center" HorizontalAlignment="Center" IsThreeState="{Binding AllowNullValue}"
                       IsEnabled="{Binding 
                 Path=EditingEnabled, UpdateSourceTrigger=PropertyChanged}"
                       IsChecked="{Binding CurrentContent, Mode=TwoWay, ValidatesOnDataErrors=True, ValidatesOnExceptions=True, Converter={StaticResource XToBoolConverter},