Remove external references to Conformer._E0

In rmgpy/statmech/conformer.pyx, _E0 is the private attribute backing
the public E0 property (to support an Energy cast on setting).
Respect that privacy and use the public interface.
Merge pull request #214 from wking/no-_E0

Remove external references to Conformer._E0

In rmgpy/statmech/conformer.pyx, _E0 is the private attribute backing the
public E0 property (to support an Energy cast on setting). Respect that
privacy and use the public interface.