added exchange_rank to allow tomopy to process exchange1, exchange 2 etc...

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
ops ... I did not make the pull request agaist the latest version ... I misses / in both dark and theta. These are now corrected and tested. I will close this pull request and open another with the correct version to compare