forces/pair: add weight parameter for auxiliary variables

In the case of different particle instances it can be useful to
calculate the full potential energy (and not only half of it) for one
particle instance (AB) and disabling it completely for the other (BA).

This commit closes #265536
cmake: make potentials (except lennard_jones) optional

As each potential increases the build time significantly, make them
optional. This is especially useful for development, as in most cases
only one potential is actually required.

Always keep the Lennard-Jones potential because it is required by the
thermodynamics, local_r4 and the verlet_nvt_hoover unit tests.
cmake: use one list to switch potentials on or off

As the number of potentials grows, a single checkbox for each one will
clutter the CMake configuration file. Instead we use a single list which
is used to define non-cached variables HALMD_WITH_${potential} in the
top-level CMake script.

Drop the special treatment of the Lennard-Jones potential. Disable tests
which link against this potential. (Script-based tests will fail if
LJ is disabled.)

lua: Copy only these potential modules which are enabled

doc: FIXME The doc still resorts to the Lua files in the source tree and
     not in the build tree. (I suppose that the order of execution of
     the various CMake subdirectories is undefined.) Thus the
     documentation will still list disabled potentials as well.