[test][difference L/L] add a couple more test cases of geometries with spikes
[copyright headers] unify the way they look
[algorithms][overlay] allow for spikes in output likestrings; this modifies the behavior of existing set ops and allows to support linear geometries with spikes;
change copy_segments for linestrings to take a template parameter to control whether spikes are accepted or not; by default they are not accepted
(but the follow code now explicitly allows for them in places where it wasn't;