binning: revise Lua module

Move C++ module out of host/gpu namespaces to simplify access from Lua.

Document usage and parameters.
neighbour: revise Lua module

Move C++ module out of namespace gpu/host in the Lua bindings to
simplify usage in Lua.

Document usage of Lua module.
max_displacement: revise Lua module

Simplify access from Lua and document this module.
lattice: revise Lua module

Document the module and simplify access to C++ instances of
the lattice module.
lua/observables/correlation: provide example usage

Add an example that illustrates the usage of user-defined correlation
functions.