DATE     94/01/10
    Opérateur ACTIVE
    ----------------

     CH1  =  ACTIVE  | GEOM   CH2  CH3  CH4  ( FLOT1 )  (MOT1) ;
                     |
                     | SECA   CHPO1 CHPO2 CHPO3 CHPO4 ;

    Description :
    _____________

    The ACTIVE operator is used for accelerating the convergence
 of non-linear calculations.


    Contents :
    __________

    Two types of acceleration are available :

    a) GEOMETRICAL :

        From 3 successive fields CH2, CH3, CH4, the 
        field CH1 is created, supposing that these 3
        fields are in geometric progression.
        If it is a matter of CHPOINTs, all the components
        are accelerated.
        If it is a matter of MCHAMLs, only the component
        named MOT1 is accelerated (this name is useless
        if the field has a single component)
        There will be an acceleration only if the convergence
        rate is smaller than FLOT1, equal to 0.98 by default
        (FLOTTANT type)
        
        
        
    b) SECANT :
     
    A vector CHAMP1 is calculated (CHPOINT type) (it is an accelerated
 estimation of CHPO3) from 4 fields (CHPOINT type) which are, in order :

           CHPO1 :  displacement increment at the first iteration
           CHPO2 :  displacement increment at the n-1 iteration
           CHPO3 :  displacement increment at the n iteration
           CHPO4 :  increment of non-linear correcting forces at
                    iteration n