Check if input_bau.gdx is given or really needed; a couple of minor changes mostly related to MAgPIE coupling

[dklein-pik]

Purpose of this PR

This PR contains a couple of minor changes to R scripts mostly related to REMIND-MAgPIE coupling (code cleaning, adding comments, ...).

The most important change and not related to coupling is a new check in checkFixCfg.R that throws an error if an NDC is to be started but no input_bau.gdx was given. In case no NDC is to be calculated but an input_bau.gdx is provided it will be set to NA since it would otherwise introduce an unnecessary dependency to the preceeding BAU run. This is most relevant for the parallel mode of coupled runs, where in each iteration the unnecessary dependency may prevent the start of subseqent runs if the preceeding BAU run (that is actually not required in a non-NDC scenario) fails.

Type of change

• New feature

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