distance_columns extracts columns from the distance matrix.
distance_columns(distances, column_indices, row_indices = NULL)A distances object.
An integer vector with point indices indicating which columns to be extracted.
If NULL, complete rows will be extracted.
If integer vector with point indices, only the indicated
rows will be extracted.
Returns a matrix with the requested columns.
If the complete distance matrix is desired, distance_matrix is
faster than distance_columns.