Default config file testing, and comment out settings

In #285 (closed)

arti_defaults.toml: this duplicates the default settings baked into the code. This is kind of inevitable if we want to supplyy an example, but there is a big risk of divergence. Either (a) there should be a test case that the defaults match the baked-in ones or (b) there should be no baked-in ones and instead things should be read from this file. Also, it is in danger of being taken as an example config file, which is not great if we ever want to change the defaults. Suggest we comment out every setting. (The test case or run-time defaulting will now need to use a mechanically-uncommented versions.)

We should:

• Comment out all of the settings in the in-tree copy of the file, so that it can be used as a "doc commented example" file
• Have a test case which uncomments the settings and checks that the result can be parsed and is the actual default (we already have a test case that parses the example, I think)
• Check that no settings from the example were ignored (blocked on #417 (closed))
• Somehow have an automated check that every setting honoured by the Deserialize impl is also in the default config file (complicated) (done in !546 (merged))