Multi-processing for duplicate elimination to speed up building of pi groups

[maxiimilian]

Hi guys,

Thanks for developing this great tool! I made a few improvements, which I want to offer to contribute back:

• logging outputs so the user can see which steps have been performed (logging level needs to be set to INFO).
• acceleration of deduplication using multi-processing with a progress bar (huge improvement!)
• formatted code using black to meet Python formatting conventions
• __flagged_var_max_sets can now be configured by the user during construction of the BuckinghamPy object, e.g., BuckinghamPy(var_max_sets=25). No code change required

Please let me know if you have any questions.
Max