v2.6.1 Now with clone() and do_copy()

This release fixes minor bugs and adds expected UVM functionality:

• Now do_copy() works as expected in a uvm_object and you can use it to enable copy() and clone(). However I recommend using the Python copy module instead
• run_test() did not clear out the singletons properly and was losing the uvm_root() singleton
• run_test() now sets the default logging level to INFO unless you pass keep_singletons=True
• The example had a bug where it did not catch errors. Now it does and demonstrates errors.