save random seed from <random_seed>system_clock</random_seed>

[rheiland]

When a model specifies system_clock to provide the RNG seed, it would be nice to have the seed value saved for later attempts at reproducibility (primarily if a single thread is used).

[drbergman]

100% agreed.

[drbergman]

Are we ok just writing a new file output/random_seed.txt that holds it? Otherwise, this will require a bit more restructuring to achieve.