change looping behavior on clusters

[felixmusil]

The change of the definition the N body order cluster has to change the way we implement and interact with the iteration over neighbors and triplet...

Now the clusters are definined with respect to the center, i.e. within its cutoff. So we can build quadruplets without triplets for instance. Also we can iterate over the quadruplets skiping triplets.

• [ ] understand what changes are needed for clusterRef (now it is built in a hierarchy where it depends on the previous order)

• [ ] implement and make the changes so that we can do atom.get_quadruplet() in the loop.

• [ ] a lot of changes in the examples, tests, calculators etc.

[felixmusil]

I started to work on it in the draft PR #171