uvm_void : example of abstract class in uvm

1. The uvm_void class is the base class for all UVM classes.
2. uvm_void is an abstract class with no data member or function.

3. It allows for generic containers of objects to be created, similar to a void pointer in the C programming language

4. User classes derived directly from uvm_void inherit none of the UVM functionality, but such classes may be placed in uvm_void-typed containers along with other UVM objects.