lua: use particle.memory for rng construction

The construction of the random number generator needed the memory
parameter if particle.memory is set to "host" when a gpu is available.

Also, make use of utility.assert_kwarg() in the updated modules.
lua/verlet_boltzmann: rename module to verlet_nvt_boltzmann

In order to be consistent with the name scheme that includes the
ensemble description, rename the module to 'verlet_nvt_boltzmann'.