get_param_value
Description:
Retrieve the parameter value associated with a named key.
If multiple values are stored against the key, only the first value is returned. If no value is associated, NULL is returned
Parameters:
| this | 
           an Entity containing the parameters  | 
      
| key | 
           the name of the key  | 
      
Returns:
| 
           the value associated with the key, or NULL  |