111 add kernel based sii approximator (closes #111, #32)

[FFmgll]

Complete rework of all regression approximators with new sampling class. Added KernelSHAP-IQ and Inconsistent KernelSHAP-IQ. Complete rework of SHAP-IQ approximator. New MonteCarlo approximator class, unifies SHAP-IQ and SVARM-IQ and based on novel sampling class. Includes the following changes:

• RegressionSII changed to InconsistentKernelSHAP-IQ
• New MonteCarlo class, includes SHAP-IQ and SVARM-IQ (and two more configurations)
• KernelSHAP-IQ introduced in Regression class
• Added new kADD-SHAP estimator (similar to InconsistentKernelSHAP-IQ but returns kADD-SHAP values)
• Several new approximation tests added

[FFmgll]

tests are probably failing sometimes due to #141