get a flag for indexing based on values (ange of fixed values).
        
        :parameter param: variable name
        :parameter range: numpy array defining the range of the values. If size(range) == 2 :
        
           * flag is computed between min and max values of range
           * flag is computed based on equality to range value.

def slice(self,param,range,surf=False):
        '''
        get a flag for indexing based on values (ange of fixed values).
        
        :parameter param: variable name
        :parameter range: numpy array defining the range of the values. If size(range) == 2 :
        
           * flag is computed between min and max values of range
           * flag is computed based on equality to range value.
              
        '''
        if np.size(range) == 2 :
            if not surf : fg = (self.__dict__[param] >= np.min(range)) & (self.__dict__[param] < np.max(range))
            else : fg = (self.__dict__[param+'_surf'] >= np.min(range)) & (self.__dict__[param+'_surf'] < np.max(range))
        elif np.size(range) == 1 :
            if not surf : fg = (self.__dict__[param] == range)
            else : fg = (self.__dict__[param+'_surf'] == range)
        else : self.Error('Range array must have 1 or 2 elements max')
        return fg