Make unit cell vector components that are almost zero exactly zero.

Force unit cell vector components that are close to zero (and should be formally zero) to zero exactly.  Some codes appear to compare vector components to zero as part of its validation.

Also fixes a minor typo.
This [isn't](https://travis-ci.org/rmcgibbo/mdtraj/jobs/31322500) vectorizing correctly.  I think it should instead be
```
a[np.logical_and(a>-tol, a<tol)] = 0
b[np.logical_and(b>-tol, b<tol)] = 0
c[np.logical_and(c>-tol, c<tol)] = 0
```