In Acts version 15.0.0, we see support being added for dynamic bins in
the bin finder. However, this also means that we no longer have useful
default constructors for this class. This has caused me a few issues in
terms of backwards compatibility. I think it would be a good idea to
reintroduce some sane default configuration for the bin finder, and I
discussed with @LuisFelipeCoelho that having an empty list of z
neighbours with a single phi neighbour would be a good default.