Add Sobol Index Method for all Grid Types

[mkstoyanov]

Anisotropic coefficients are a good estimate for Global/Sequence/Fourier grids, but Local Polynomial and Wavelet grids do not have a good estimate.

• the Sobol indexes can be estimated using Monte Carlo method easily, but it requires user code
• look into analytic estimates of the indexes
• if not analytic, then add the Monte Carlo method as an addon