Sort spikes in spiketrains 

Example: 
```python
import neo 
import quantities as pq 
st = neo.SpikeTrain([1, 0, 2, 3]* pq.ms, t_start=0*pq.ms, t_stop=3*pq.ms)
print(st) 
>>> [ 1.  0.  2.  3.] ms
```
should give an error or correctly sort the spikes. 