fixed center of mass formula

Old code returned with this error when attempting to calculate the center of mass for a molecule:
```
>>> benzene.center_of_mass
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "chemlab/core/molecule.py", line 367, in center_of_mass
    return ((self.m_array * self.r_array)/self.m_array.sum()).sum(axis=0)
ValueError: operands could not be broadcast together with shapes (12,) (12,3) 
```