mdsim/neighbour: fix Lua assertion

Access to neighbour.defaults requires an instance of the neighbour
module. Further, the different neighbour modules and the binning module
(may) have different defaults. The binning module has an optional
occupancy parameter, i.e., it handles the value "nil" correctly.

The issue has been introduced in commit:9ff37eb (following a suggestion
by myself).

Finally, I noted that binning can not be disabled (in contrast to what
the code suggests). Constructing a neighbour module with the argument
"disable_binning=False" throws upon construction of the Hilbert sort
module.