* fixed typos

* fixed several typos

* Corrected test_compare_agents() function

TrivialVacuumEnvironment was missing a pair of parenthesis while creating the "environment" object of the TrivialVacuumEnvironment class
ModelBasedVacuumAgent and ReflexVacuumAgent were also missing parenthesis while creating the "agents" object.

* Reverted changes made to test_compare_agents()

* fixed typo

* Rewrote parts of search.ipynb

* Fixed typo and cleared cell output

Partially solves #687

* DataFile Update

* Update learning.py

* Update search.py

* Update test_learning.py

* Update test_text.py

* Add files via upload

* Update text.ipynb

* Update search-4e.ipynb

* Create CONTRIBUTING.md