SeqRes missing from substructures

The seqres of a partial substructure is always empty. For instance,

``` java
Structure s = StructureIO.getStructure("1hh0.A");
List<Group> seqres = s.getChain(0).getSeqResGroups();
assertEquals("Wrong seqres length",46,seqres.size());
```

fails with  `expected:<46> but was:<0>`.

The problem is that `SubstructureIdentifier.reduce()` doesn't handle the seqres properly.
