Split API into component-based classes and files

[Manav Sehgal]

Split API methods into component-based classes and files based on where the methods get applied in a Machine Learning project.

• Class Base for shared variables and methods.
• Class Speedml for methods which apply across multiple components.
• Class Feature for methods performing feature engineering.
• Class Plot for methods plotting and charting.
• Class Xgb for XGBoost wrapper methods.

[Manav Sehgal]

Done in commit 583aad