lua/potentials: add label to profiler output

In order to differentiate between the potential instances, add the label
to the description, if available.

Also fix the instance labels to their correct name, as most of them were
named "lennard_jones".
max_displacement: use cache in order to track position updates

Using the position cache reduces the number of calculations for the
maximum displacements if the module called multiple times within the
same MD step, e.g. when used by multiple neighbour modules.
lua/neighbour: add optional 'displacement' parameter, simplify 'r_cut'

Using a user constructed displacement module, one can share instances of
max_displacement across multiple neighbour modules.

Further, the argument 'r_cut' can be given as number which will be
converted into a matrix automatically.
lua: add instance labels to profiling output

Add the particle instance label to the profiler output to binning,
max_displacement, particle and hilbert modules.