checkCIF procedure

PLAT031

 This test checks whether a refined extinction parameter is meaningful  
 i.e. whether its value is significantly larger than its corresponding s.u. 
 If not, this parameter should be removed from the model and the structure  
 refined without this meaningless additional parameter. 
 The current default gives a warning when its value is within 3.33 s.u. 
 SHELXL97-2 will not allow negative values leading to ill-convergence and   
 non-zero maximum shift/error values: remove the extinction parameter from the  
 refinement.