Added exchange_rank to direct tomopy to the dataset to reconstruct

We are prepocessing the raw txm data to generate realigned projections. Once done with the preprocessing we are saving the new data in the same Data Exchange HDF5 under \exchange1. This modification will allow to point tomopy to reconstruct \exchange1
It is probably better to do this just once and stash the value

```
if exchange_rank > 0:
    exchange_base = 'exchange{:d}'.format(exchange_rank)
else:
    exchange_base = 'exchange'
```

This way if you decide later to tweak the format you only have to change it one place, not 4