Foil (#946)

* Modified FOIL_container

* Added unit tests for FOIL_container functions

* Added knowledge_current_best notebook

* Added knowledge_FOIL notebook

* Added knowledge_version_space notebook

* Added images for knowledge_FOIL notebook

* knowledge.ipynb replaced by knowledge_current_best.ipynb, knowledge_version_space.ipynb, knowledge_FOIL.ipynb

* modify knowledge.py