Add shutil.rmtree wrapper that works better on Windows

[dangunter]

Due to per-process file locking in windows, shutil.rmtree sometimes fails where it would succeed a second or three later, once Windows thinks the process is done. A utility function that did this would be very useful in projects that use Pyomo/pyutilib, such as IDAES.

[dangunter]

e.g. a function of the flavor:

def rmtree_hard(*args):
    for i in range(3):
        try:
            shutil.rmtree(*args)
            return True
        except:
             time.sleep(1)
    return False