Implement local kernel methods

[rmcaixeta]

• Interface to kernel functions
• Implement the experimental variography that assign weight to pair of values according their distance to the reference coordinate. Closer to reference point -> higher weights. It is useful to model variograms of different regions of a domain.

Reference: Machuca-Mory, D. F., & Deutsch, C. V. (2013). Non-stationary geostatistical modeling based on distance weighted statistics and distributions. Mathematical Geosciences, 45(1), 31-48.