Support reproducible restarts

[Sbozzolo]

RRTMGP uses a random number generator. If we want to restart a run and debug it in a deterministic way, we need to control how random numbers are generated.

The simplest way I can think of is to force the seed to something deterministic before writing the restart file. The seed could be anything (even time) and could be stored to the restart file, so that we can reproduce the same stream of random numbers.