[bug] get rid of all default seeds in different configuration sources

There are currently 3 "seed" options:

1) A "seed" for simulations
2) A "seed" for preprocessing (this mostly controls randomization of
dataset splits)
3) A "seed" for training

All of these had default values in the default config files.  I think
partly as an artifact of when I ported over some of Jean and Theophile's
old config files into the code.

In practice, users should expect stochasticity by default.  The schemas
have a default value of "null" for all these seeds, which is equivalent
to random seeding of the PNRG.  If users want to set a specific seed for
reproducibility they should do so manually.

(Possible future enhancement: Record the seed that was used so they can
reproduce the same run even if the seed was not set explicitly first.)