[convex_hull] Fix invalid number of Points generated for open OutputGeometry.
[test][convex_hull] Add tests for open output Geometries.
[convex_hull] Generate consistent results for closed and open Geometries.

The result differ only for the last Point duplicated for closed.
This code should be faster than the original since any condition isn't checked inside a copying loop and the end iterators are created only once.
[test][convex_hull] Add test for MultiPoint for which convex_hull() should return invalid Polygon.
[convex_hull] Generate "proper" number of Points also when invalid Output is generated.

E.g. for 1- or 2-Point MultiPoint input the Output is a 0-area Ring (Point-like or Segment-like respectively). After this change it should contain "proper" number of Points (3 for open, 4 for closed). The additional Point is added by duplication of the first one.
[test][convex_hull] Update convex_hull tests.

Update multi_convex_hull test to reflect recently made changes for Point-like and Segment-like Output.

Remove the test for MultiPoint from convex_hull test since it's already tested in multi_convex_hull test.
Merge pull request #152 from awulkiew/fix/convex_hull

Fix/convex hull