Utility function to see if an object has a particular class.
Useful when we don't know how many other classes the object may
have.
     
    
    
    Arguments
- x
 
the object we want to check the class of.
 
- cls
 
the class we want to know if it has or not.
 
 
    
    Value
    TRUE if the object has the class.
FALSE if the object does not have the class.