lua/modified_lennard_jones: revise and document module

Use the HALMD 0.3.x scripting interface and add a documentation of the
module and its attributes. Also make use of 'memory' attribute of
the particle module.
lua: fix usage of device variant when 'memory' should be used

binning, neighbour and verlet_nvt_hoover should use the memory parameter
of the given particle module instead of the implicit detection via
device.gpu