add_function
Description:
Adds a function symbol which can not perform short evaluation.
Use add_ifunction for functions which can.
Parameters:
| this | 
           a SExp  | 
      
| scope | 
           a scope  | 
      
| name | 
           a function name  | 
      
| func | 
           a function callback  | 
      
| user_data | 
           user data for   |