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Short helpfile for action REFERENCE_GRID
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The following arguments are compulsory: 

               GRID_MIN - ( default=auto ) the lower bounds for the grid 
               GRID_MAX - ( default=auto ) the upper bounds for the grid 
               PERIODIC - are the grid directions periodic 
                   FILE - the name of the file that contains the reference data 
                  VALUE - the name of the value that should be read from the grid 
                   FUNC - the function to compute on the grid 
               GRID_BIN - the number of bins for the grid 
           GRID_SPACING - the approximate grid spacing (to be used as an alternative 
                          or together with GRID_BIN) 
                    VAR - the names to give each of the grid directions in the 
                          function. If you have up to three grid coordinates in your function 
                          you can use x, y and z to refer to them. Otherwise you must use 
                          this flag to give your variables names. 


