get the list of classes inheriting a given super class provided by its name
     
    
    getClassesInheriting(classname, extended, pretty)
 
    
    Arguments
- classname
- the name of the superclass for which inheriting sub-classes have to be listed 
- extended
- whether we want to look at user namespace for third-party sub-classes 
- pretty
- prettify the output as - data.frame
 
 
    
    Examples
      getClassesInheriting("ISAbstractObject")
#> character(0)