negative phonon energies from phonopy calculation

phonopy may give negative phonon energies. Right now they are [forced to be zero](https://github.com/mcvine/phonon/blob/4a7d049c214728273b0b3840917f669a76499c3b/phonon/from_phonopy/__init__.py#L90). Should we try different strategies like shifting all energies up like this:

```
    min = np.min(freq)
    if min < 0:
        freq += -min
```
?
