Distance Functor now merged with latest master in sb/bugfixes

[Sylvain Bougerel]

Hi,

In the branch sb/bugfixes (no mistakes this time) I've merged the
changes for the distance functor with the changes from master.

test_kdtree.cpp and test_hayne.cpp are the version from the master,
they both compiled and passed without any modification.

So far the only distance functor tested is euclidean_distance. I
haven't tested manhattan_distance or fast_euclidean_distance yet.

I have added fast_euclidean_distance that is unsafe because it can
overflow, but will compute faster than the safer euclidean_distance.
However it is not the default distance functor for the tree, so you
need to declare your templates accordingly if you intend to use it.

Cheers,

Sylvain