code: public static <R, C, V> Cell<R, C, V> immutableCell(
        @Nullable R rowKey, @Nullable C columnKey, @Nullable V value) {
      return new ImmutableCell<R, C, V>(rowKey, columnKey, value);
    }
docs: Returns an immutable cell with the specified row key, column key, and
  value.
  
  <p>The returned cell is serializable.
  
  @param rowKey the row key to be associated with the returned cell
  @param columnKey the column key to be associated with the returned cell
  @param value the value to be associated with the returned cell