Simplify `linmod.data.main`

[thanasibakis]

We could instead do something like:

def main(cfg: Optional[dict]):
    if cfg is None:
        cfg = DEFAULT_CONFIG
    else:
        cfg = {"data": DEFAULT_CONFIG["data"] | cfg["data"]}

or perhaps even just set a default argument value of cfg = DEFAULT_CONFIG and modify the argparse logic below to never pass None

Originally posted by @thanasibakis in #73 (comment)