Refactoring (removed owlapy), verbalization, documentation update and some fixes

[alkidbaci]

Refactoring of directories:

• According to issue Refactoring: dice-group/ontolearn/owlapy into dice-group/owlapy #309 part of owlapy directory are now moved to the separate repo https://github.com/dice-group/owlapy.
• Subdirectory owlapy/owlready2 remains in Ontolearn and is renamed to base. Full path: ontolearn/base.
• owlapy/ext also remains in Ontolearn. Full path: ontolearn/base/ext.
• Removed directory ontolearn/core and moved it's scripts to ontolearn/base.
• owlapy v0.1.0 is now a dependency and all references are updated.

Now its possible to verbalize generated hypotheses #292 :

• Added static method verbalize for concept learning models.
• Added example/verbalization_example.py script that demonstrates it.
• Important Notes:
  - Not all class expressions can be visualized mentioning here especially those who contain data property restrictions.
  - deeponto is not a required package, therefore users have to install it before trying verbalization.
  - graphviz is also required to be downloaded before using verbalization. (links are in the example file & documentation)

Removed use_triplestore arggument:

• Now only specifying the path of the triplestore to KB means that you will be using triplestore for the learning/reasoning processes.

Documentation Update:

• Changed reference of owlapy classes to link to owlapy repo.
• Changed a the description for concept learners, made it a bit less complicated.
• Added expressiveness for each concept learner Add expressiveness of CEL algorithms to docs #306
• Rearranging and some other few changes and here and there.

Removed some unused files