get_fraction
Description:
Sets the integers pointed to by value_numerator and value_denominator corresponding to the value of the given field.
    
Caller is responsible for making sure the field exists and has the correct type.
Parameters:
| this | |
| fieldname | 
           the name of a field  | 
      
| value_numerator | 
           a pointer to an int to set  | 
      
| value_denominator | 
           a pointer to an int to set  | 
      
Returns:
| 
           true if the values could be set correctly. If there was no field with   |