velocities/boltzmann: add method to change the temperature
integrators/verlet_nvt_boltzmann: add method set_temperature()
integration test: fix modularity test "two_particles"

The test still fails since
- neighbour/from_binning does not work for small systems
- nothing is eventually checked

There are several workarounds in the scripts, marked by FIXME, which we
should address at a later point in time.
test: fix ctest_linking

As the version string of HALMD includes some more special characters,
escape these, too. Note that escaping the ']' is not possible, use
workaround from http://public.kitware.com/Bug/view.php?id=11370