Create q_full_adder.py (#6735)

* Create q_full_adder.py

This is for the #Hacktoberfest.
This circuit is the quantum full adder. I saw that in the repo is the half adder so I decided to build the full adder to complete the set of adders.
I hope that this is enough to be consider a contribution.
Best,
Kevin

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* Erase the unused numpy library

* Create the doctest.

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