flat_map
Description:
Returns a seq which contains the elements of the results of applying the given mapper function to the elements of this seq.
This is a stateless intermediate operation.
Parameters:
| mapper | 
           a non-interfering and stateless mapping function. if it returns an Gee.Iterator.valid iterator, the element that the iterator points is also included in the result.  | 
      
Returns:
| 
           the new seq  |