benchmarks: revise simulation scripts

Remove 'force' parameter from integrators.

Rename default output files.
neighbours: update list on reverse tag update of particle2

If the reverse tags (i.e. the ordering of the particle indices) of the
second particle instance changed, the neighbour lists where not updated,
although this is required as the mapping of array index<->particle index
becomes invalid.
Fix this by adding the reverse_tag_cache of particle2 to the neighbour
list cache.

Note that gpu/from_binning is not updated, as it does not support two
particle instances (yet).
lua/binning: provide 'particle' attribute

Make it possible to read out the current particle instance from the
binning module.

Also use default value skin parameter, if not given.
lua/neighbour: allow binning modules to be given as parameter

If one has more than neighbour list modules that share a common particle
instance, it will be useful to reuse the binning modules.
gpu/binning: add cell_size() member function

This function returns the current number of placeholders in the cell
list and is more explicit than getting it from the grid dimensions of
the binning module.
gpu/neighbours: support multiple particle instances in from_binning

Introduce a naive neighbour list implementation that does not make
assumptions about the block size of the GPU kernel. This allows to
support different particle instances to work with from_binning.

Also extend the previous implementation to support two particle
instances -- this works only if both particle instances have the same
number of binning cells in each spatial direction and the number of
placeholders per cell of the binning modules match.
neighbour: support specification of GPU implementation

As the implementations of the from_binning kernels have different
performances on different GPUs, give full control over the used
implementation to the user.

Due to the limitations of the "shared_mem" implementation, the "naive"
implementation takes precedence, if required.
gpu/neighbours: make 'logger' parameter last parameter

All modules have the logger parameter as the last parameter, this is
just for consistency.
gpu/binning: remove unused variable