Allow appeding to cfg$output via scenario_config.csv; allow setting output also for coupled runs; update comments

[dklein-pik]

Purpose of this PR

Fixes #1451 and closes remindmodel/development_issues/issues/247

Type of change

• Bug fix

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• I adjusted the reporting in remind2 where it was needed
• I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• All automated model tests pass (FAIL 0 in the output of make test)
• The changelog CHANGELOG.md has been updated correctly

[orichters]

Hope nobody gets confused later why once unlist and once [[1]] is used, but that might be a minor detail. Thanks for fixing it.

[dklein-pik]

Hope nobody gets confused later why once unlist and once [[1]] is used, but that might be a minor detail. Thanks for fixing it.

Harmonized this.