[Enhancement] Speed-up directional variogram calculations

[SimonMolinsky]

Is your feature request related to a problem? Please describe. Directional variogram calculations are relatively slow. They should be faster. The two things can be done:

• [ ] Instead of the elliptical area for the neighbor search, use triangular approximation;
• [ ] Re-write a function for multi-core processing.

Describe the solution you'd like At this point, the triangular approximation is a better step to take.

Describe alternatives you've considered n/a

Additional context n/a