ConfigOutput only generates output if ``start`` option is set

[einola]

The ConfigOutput module will not generate any output if the ConfigOutput.start option is not set. The expectation is that not setting ConfigOutput.start is equivalent to setting it to the model start (Model.start).

To reproduce, pick a favourite config file (e.g. config_column.cfg) and remove the ConfigOutput.start option.

[timspainNERSC]

The default value of the start time should be the model start time. I'm sure I wrote code to do this in the past. The help text even says that model.start is the default value. I'm not sure what happened to that code.

    map[pfx] = {
        …
        { startKey, ConfigType::STRING, {}, "model.start", "",
            "Date at which to start outputting data." },
        …
    };