Method is used for appending the given Fields instance to this instance, into a new Fields instance suitable for use as a field selector.
That is, any positional elements will be retained during the append. For example, the {@code 5} and {@code 0}are retained in the result.
{@code assert new Fields( 5, "a" ).append( new Fields( 0, "b" ).equals( new Fields( 5, "a", 0, "b" )}
Note any relative positional elements are retained, thus appending two Fields each declaring {@code -1}position will result in a TupleException noting duplicate fields.
See {@link #subtract(Fields)} for removing field names.
@param fields of type Fields
@return Fields