issue #192: decouple visualisation from core explainability

[christinaexyou]

This PR address #192 by moving viz methods from the explainers module to following files:

• visualizations/visualization_results.py: abstract class for visualization methods
• visualizations/factory.py: gets visualization according to explanation method/class
• visualizations/shap.py
• visualizations/lime.py
• visualizations/pdp.py
• visualizations/distance.py

[christinaexyou]

changes made:

• moved all viz methods to seperate module visualizations
• updated tests to reflect change

[ruivieira]

LGTM.

I just added a comment inline about type hints for plot.
Also, perhaps the whole factory could go into visualizations/__init__.py to simplify the import (i.e. from trustyai.visualizations import plot).

But I'm fine with merging without these.

[christinaexyou]

changes made:

• renamed visualizations/factory.py -> visualizations/__init__.py
• refactored tests to reflect change