The standalone MaD generator now uses `0` for threads and throttles the
RAM to use 2GB per thread by default.
Also, replaced the hand-written argument parsing with `argparse`.
* fix a bug where the order of model generation was determined by the
order in the `download.json` file of the experiment rather than the
order in the config file
* allow configuring `--ram` and `--threads` in the MaD generator scripts
* use no `--ram` and `--threads=0` by default in the bulk generator
(single generator defaults are left unchanged)
* allow to pass `--dca` multiple times, taking DBs from experiments
listed last. This allows to run a subset of the sources in a "fixup"
experiment and use it to "patch" a previous run without rerunning
everything.
