SimTk/msmbuilder — pull request #74 by rmcgibbo (2012-10-15 23:51:41+00:00)
Title: AtomPairs should complain nicely when required argument is not passed

I got bored of seeing this error

```
atom_pairs = np.array(atom_pairs, dtype=int)
TypeError: long() argument must be a string or a number, not 'NoneType'
```

which is caused by not supplying a required keyword argument to the AtomPairs constructor. This PR makes the error message more meaningful.