Add year to published reactions

The year of published reactions is now stored during script executions and
stored in the datafile.
Remove SMILES difference tracking

In case of published reactions which were regenerated by retrosynthetic
transforms the script was storing both reaction SMILES, the original and
recreated one. As we do not need them at the moment, I removed the code
responsible for doing this.

For future reference , it is better to save this information in class
instances and not in the main script as it clutters in unnecessarily.
Restore some comments

In some comments 'stats' were changed to 'status'. But the relevant data
structures represents *statistics*, thus I revert them to the original
version.
Revert rxnpca.py docstring to original form

The new docstring describing the purpose of the script was focused on linear
discriminant analysis. But it contradicts the script name and does not reflect
its goal. Thus, I revert to earlier description.