MAgPIE coupling: use new MAgPIE variable names; allow negative values for n2ofertsom (tiny fraction of total n2o)

[dklein-pik]

• Replace old MAgPIE cost variable with new one.
• in coupling script: remove filtering of negative LU emissions (other than co2luc). Only n2ofertsom is expected to get negative, which is tolerated by REMIND from now on
• tolerate negative values for n2ofertsom and deactivate MAC for this tiny fraction of total land use n2o (as agreed in coupling meeting https://gitlab.pik-potsdam.de/piam/remind-magpie-coupling/-/wikis/Home/Meetings)

Purpose of this PR

Prepare REMIND for new MAgPIE intput data.

Type of change

(Make sure to delete from the Type-of-change list the items not relevant to your PR)

• Bug fix
• Refactoring
• New feature
• Minor change (default scenarios show only small differences)
• Fundamental change
• This change requires a documentation update

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)

Further information (optional):

• Test runs are here:
• Comparison of results (what changes by this PR?):