Minor update - DEM smoothing improved

This version includes the following minor improvements:

1. DEM smoothing buffer added to allow observations to be on the boundary. Buffer grid cells are excluded from the kriging.
2. DEM smoothing kernel grid cell size added as a user input.
3. DEM smoothing kernel size increased from 5x5 to 11x11 grid cells.
4. DEM smoothing undertaken using user set number of cores.
5. Documentation improve to list all function inputs.