Fix for unify algorithm in logic.py (#1101)

* Fix issue #1053

Unify algorithm fixed by performing a perform a cascade substitution when a new mapping is added
This issue was already known and fixed in the aima-java repo.

* added two more test in test_logic.py
updated documentation for cascade_substitution function in logic.py

* Fixed brackets missing in test_logic.py for the new test

* Fixed typo error, missing space and double quotes for docstrings

* comments changed to cascade_substitution function in logic.py