Adding new attributes and updating `update_neighbors` method.
Adding new attributes and updating `update_neighbors` method.

Adding new attributes `neighbors` and `neighbor_distances`
to `KDTAtoms` class and updating the `update_neighbors`
method to set the `neighbor_distances` attribute
for each `KDTAtom` in self.

androomerrill/scikit-nano,scikit-nano/scikit-nano,scikit-nano/scikit-nano,androomerrill/scikit-nano