Update atomindices.py

```
Traceback (most recent call last):
  File "/home/kyleb/opt/bin/hmsm", line 5, in <module>
    pkg_resources.run_script('mixtape==0.1', 'hmsm')
  File "/home/kyleb/opt/lib/python2.7/site-packages/pkg_resources.py", line 505, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/home/kyleb/opt/lib/python2.7/site-packages/pkg_resources.py", line 1245, in run_script
    execfile(script_filename, namespace, namespace)
  File "/home/kyleb/opt/lib/python2.7/site-packages/mixtape-0.1-py2.7-linux-x86_64.egg/EGG-INFO/scripts/hmsm", line 8, in <module>
    app.start()
  File "/home/kyleb/opt/lib/python2.7/site-packages/mixtape-0.1-py2.7-linux-x86_64.egg/mixtape/cmdline.py", line 175, in start
    instance.start()
  File "/home/kyleb/opt/lib/python2.7/site-packages/mixtape-0.1-py2.7-linux-x86_64.egg/mixtape/commands/atomindices.py", line 108, in start
    out = np.array(list(itertools.combinations(atom_indices)))
TypeError: Required argument 'r' (pos 2) not found

```