Track lost samples from rarefaction

This should replace #329. Note that if you have an ipython cluster running you need to terminate and then reload after doing a pull.
excellent, that suggests a really easy solution:

```python
proc_data_id = table.metadata()[0]['Processed_id']
ids = set(table.ids())
all_samples[proc_data_id] = all_samples[proc_data_id] - ids
```