Octree Iterator begin/end method and added tests

[frozar]

I propose to delete begin()/end() iterator method definitions in class OctreePointCloud and OctreePointCloudAdjacency. As these classes derived from OctreeBase, which defines all iterator methods, it is not necessary and confusing to redefine them.

I also add some test over the OctreePointCloudAdjacency to check the basic iterator behavior (construction, comparison).

[SergioRAgostinho]

This looks totally ok. I just feel it misses that test you added more recently actually iterating through the octree, counting branch and leaf nodes, etc... Basically making sure it works. I'm ok without it nevertheless.

[frozar]

We this implementation, every test from test/octree/test_octree.cpp and test/octree/test_octree_iterator.cpp pass without any problem.

In test/octree/test_octree.cpp you have a bench of existent test which check the number of branch and leaf nodes and so on over an octree, so for me it seems OK.

Nevertheless I plan to add more tests over every kind of iterator in PR #1983