Huygens Crater

The new release with the major fix of bug related to the prediction error variance calculation. It was wrong in the previous releases of the package (but it didn't affect predictions, just error variance terms).

Changes:

• neighbors selection (lags counting) has been changed,
• TheoreticalSemivariogram searches for optimal sill in a grid search algorithm,
• corrected error in Krige class; now calculation of error variance is correct.

Now predicted values have a lower variance of predictions and they are better due to the fact, that optimal sill is derived from the grid search. (In the previous releases sill was fixed and equal to data variance).