Most output formats destroy non-standard bond information

```python
mol = mdt.from_smiles('CCCC')
print mol.write('pdb')
```

Gives
```
HETATM 1 C0 UNL 1 0.928 -0.017 -0.007 0.00 0.00 C
[...]
HETATM 14 H9 UNL 1 0.809 2.183 1.709 0.00 0.00 H
END
```

But we expect a CONECT section ... at least for the HETATMs