[concatenate iterator] re-factor unit test; add tests for bidirectional iterators;
[test][iterators] added copyright header
[test][iterators] re-factor test code for flatten and concatenate iterators
[test][point iterator] re-factor test code; add tests for points_front, points_back, points_reverse_iterator; test point_iterator as bidirectional iterator
[point_iterator] add free functions points_front and points_back that return the first and last point of a geometry
[flatten_iterator] implement decrement operation for flatten iterator
[point_reverse_iterator] implement reverse iterator for geometries