AdaptorMaxOrder should include a switch for centered/compact clusters

[mastricker]

As discussed: AdaptorMaxOrder should be able to built two types of clusters: centered and compact. Compact means that every atom is within the cutoff of every other atom. Centered means that the cluster includes all atoms that are within a sphere of the cutoff radius of the central atom.

compact: Check for distance between the added higher order atoms. centered: only add neighbours of the cluster of the central atom.

Marked this as an enhancement, since it's not technically a bug currently. And the feature, especially the compact cluster does not exist.

[mastricker]

Just for reference: this has only been partially addressed in #197. The default is currently the centred cluster type (all neighbours are within the cutoff of the central atom).

The compact cluster functionality is still missing, but that will be either be done in #171 or after that has been merged, since that PR changed the infrastructure of adaptor_increase_maxorder a little bit.