The derived type planck_rng is used by the Random Number Generation routines
rand_init,
rand_uni,
rand_gauss to describe fully the current RNG
sequence.
Most users do not need to know about the planck_rng definition. It may be
useful for those wanting to take a snapshot of the RNG sequence they are using (by eg,
dumping the latest values of planck_rng structure on disk) so that the same sequence can be resumed
later on from that same point.