Prevent arbitrary SplittableInt in lbtim.

Fixes a bug where it is possible to assign a SplittableInt with arbitrary named digit ranges to `PPField.lbtim`. For example:
```
>>> field.lbtim = SplittableInt(1234, {'foo': 0, 'bar': slice(1, None)})
>>> field.lbtim.foo
4
```

NB. If this gets merged then I'd do the same for `lbcode` and `lbpack`, and similar for `lbproc`.