Wrong sidechain atoms returned from residue after copying

The following test should pass, currently fails:

```python
mol = mdt.from_pdb('1YU8')
sidechain_atoms = list(mol.residues[5].sidechain)

m2 = mol.copy()
sa2 = list(m2.residues[5].sidechain)
for atom in sa2:
assert atom.molecule is m2
```