compute_modification_statements
Description:
Makes this try to compute INSERT, UPDATE and DELETE statements to be used when modifying this's contents.
Note: any modification statement set using set_modification_statement will first be unset
This function is similar to calling 
        compute_modification_statements_ext with cond_type set to gda_data_select_cond_pk
Parameters:
| this | 
           a DataSelect data model  | 
      
Returns:
| 
           true if no error occurred. If false is returned, then some modification statement may still have been computed  |