copy_state() returns None

[matsulib]

copy_state() returns None when self.copy_strategy is mistakenly set up.
Then the error message is not easy to understand.

For example, when I set tsp.copy_strategy = "copy" instead of "slice" in example/salesman.py,
the output was like this.

$ python examples/salesman.py
 Temperature        Energy    Accept   Improve     Elapsed   Remaining
Traceback (most recent call last):                 0:00:00
  File "examples/salesman.py", line 88, in <module>
    state, e = tsp.anneal()
  File "../simanneal/simanneal/anneal.py", line 192, in anneal
    self.move()
  File "examples/salesman.py", line 31, in move
    a = random.randint(0, len(self.state) - 1)
TypeError: object of type 'NoneType' has no len()

It took time to know where the 'NoneType' came from.

It would be useful if we can see the cause is tsp.copy_strategy.

[perrygeo]

fixed by #20