Customizable worker log level in Dallinger

[fmhoeger]

Currently the log level for the Dallinger worker process is hard-coded to 'WARN' in dallinger_scripts/worker.py.

    log_level = os.environ.get("loglevel", "WARN")

The proposal is to make a new config variable called loglevel_worker (compare with the pre-existing loglevel config variable) which defaults to 2 (i.e. WARN) to preserve pre-existing functionality but can be customised (e.g. in PsyNet) to set it to a more sensible 0 (i.e. DEBUG).