ignore_nan added to documentation CombineResultsRule

Author: wschoonveld

docs/manual/rules/combine_results_rule.md

@@ -8,11 +8,13 @@ FORMAT
       operation: <statistic_opperation_applied>
       input_variables: [<list with_input_variable_names>]
       output_variable: <one_output_variable_name>
+      ignore_nan: <boolean>
 ```
 
-The combine results rule combines the output of two or more variables to one output variable. The way this data is combined depends on the operation chosen. This could be used for adding mutual exclusive results (e.g., habitat suitability based on flow velocity and water depth) or assessing difference between results (e.g., waterlevel and bathymetry to get the water depth).The rule operates one or multiple  3D variables or 2D variables, independent of the time axes, as long as these all have the same dimensions and returns a single 3D or 2D result, either with time axis, depending on input.
+The combine results rule combines the output of two or more variables to one output variable. The way this data is combined depends on the operation chosen. This could be used for adding mutual exclusive results (e.g., habitat suitability based on flow velocity and water depth) or assessing difference between results (e.g., waterlevel and bathymetry to get the water depth). The rule operates one or multiple  3D variables or 2D variables, independent of the time axes, as long as these all have the same dimensions and returns a single 3D or 2D result, either with time axis, depending on input.
 
 Operations available: Add, Subtract, Multiply, Average, Median, Min and Max
+The parameter ignore_nan is optional and has a default value of False. When this parameter is set to True, empty values (NaN) will be ignored for all operations, except for Multiply.
 
 The rule needs to be applied to an existing 2D/3D variables with or without time axis. A new 2D/3D variable with or without time axis is created when the rule is executed.
 
@@ -26,6 +28,7 @@ The rule needs to be applied to an existing 2D/3D variables with or without time
       operation: subtract
       input_variables: ["water_level","water_depth"]
       output_variable: bathymetry_time
+      ignore: True
 
 ```