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
and just to be safe on typing maybe use
```
exchange_base = 'exchange{:d}'.format(int(exchange_rank))
```