Checking for empty states

Currently, you get an error like this when trying to use SaveStructures.py if a state has no assignments (and thus the variable `state_counts`==0)

```
  File "/home/lilipeng/epd/epd-7.3-2-rh5-x86_64/lib/python2.7/site-packages/msmbuilder-2.8-py2.7-linux-x86_64.egg/msmbuilder/project/project.py", line 299, in randomize
    result = random.randint(0, state_counts, size=size)
  File "mtrand.pyx", line 874, in mtrand.RandomState.randint (numpy/random/mtrand/mtrand.c:5970)
ValueError: low >= high
```

This might be a little more interpretable