fixate_field_nearest_fraction
Description:
public bool fixate_field_nearest_fraction (string field_name, int target_numerator, int target_denominator)
  
  Fixates a Structure by changing the given field to the nearest fraction to 
        target_numerator/target_denominator that is a subset of the existing field.
Parameters:
| this | |
| field_name | 
           a field in this  | 
      
| target_numerator | 
           The numerator of the target value of the fixation  | 
      
| target_denominator | 
           The denominator of the target value of the fixation  | 
      
Returns:
| 
           true if the structure could be fixated  |