refactor: update logging to always output to STDOUT/STDERR by default

[hollandjg]

Benefits:

• The code handling the logger initialization is much simpler. (There isn't any.)
• The command line option --log 0 or --log 1 is gone.
• We remove the need to mount the directory for logs in the dockerfile.
• It's easy to get logs using cylc, which will automatically collect the logs from stderr and provide them to the user via the user interface.
• It's still easy enough to pipe the log output to a file when needed.
• More documentation for debugging.

Unchanged:

• Adding debug output is the same as ever (it uses the environment variable JULIA_DEBUG).

Drawbacks:

• Defining the output filename for the logfiles becomes the responsibility of the user or workflow writer.

  This is going to be an idée fixe in the upcoming refactoring, as we make it possible to specify input and output filenames for each of the steps of the processing. If we're not happy with that, we should discuss it now.

Additional changes:

• add a _test-cli.sh script for running checks on the pipeline tools without needing a full cylc workflow run.